U.S. patent application number 12/041504 was filed with the patent office on 2009-09-03 for continuous process for the production of carbon nanofiber reinforced continuous fiber preforms and composites made therefrom.
This patent application is currently assigned to Performance polymer Solutions, Inc.. Invention is credited to David B. CURLISS, Jason E. Lincoln.
Application Number | 20090220409 12/041504 |
Document ID | / |
Family ID | 41013331 |
Filed Date | 2009-09-03 |
United States Patent
Application |
20090220409 |
Kind Code |
A1 |
CURLISS; David B. ; et
al. |
September 3, 2009 |
CONTINUOUS PROCESS FOR THE PRODUCTION OF CARBON NANOFIBER
REINFORCED CONTINUOUS FIBER PREFORMS AND COMPOSITES MADE
THEREFROM
Abstract
This invention provides a continuous process for the growth of
vapor grown carbon fiber (VGCNT) reinforced continuous fiber
preforms for the manufacture of articles with useful mechanical,
electrical, and thermal characteristics. Continuous fiber preforms
are treated with a catalyst or catalyst precursor and processed to
yield VGCNT produced in situ resulting in a highly entangled mass
of VGCNT infused with the continuous fiber preform. The continuous
process disclosed herein provides denser and more uniform carbon
nanotubes and provides the opportunity to fine-tune the variables
both within an individual preform and between different preforms
depending on the characteristics of the carbon nanotubes desired.
The resulting continuous fiber preforms are essentially endless and
are high in volume fraction of VGCNT and exhibit high surface area
useful for many applications. The invention also provides for
composites made from the preforms.
Inventors: |
CURLISS; David B.;
(Beavercreek, OH) ; Lincoln; Jason E.; (Englewood,
OH) |
Correspondence
Address: |
FULBRIGHT & JAWORSKI L.L.P.;Attn: MN IP Docket
600 Congress Avenue, Suite 2400
Austin
TX
78701
US
|
Assignee: |
Performance polymer Solutions,
Inc.
Centerville
OH
|
Family ID: |
41013331 |
Appl. No.: |
12/041504 |
Filed: |
March 3, 2008 |
Current U.S.
Class: |
423/447.2 ;
422/149; 423/447.1; 977/742; 977/843 |
Current CPC
Class: |
B82Y 30/00 20130101;
D01F 1/10 20130101; B01J 37/0203 20130101; B29B 11/16 20130101;
B01J 6/008 20130101; B01J 23/75 20130101; C08J 5/005 20130101; B01J
19/22 20130101; C08J 2300/00 20130101; B01J 37/084 20130101; B01J
23/755 20130101; D01F 9/127 20130101; B01J 35/06 20130101; B01J
23/745 20130101; C08J 2379/08 20130101 |
Class at
Publication: |
423/447.2 ;
423/447.1; 422/149; 977/843; 977/742 |
International
Class: |
D01F 9/127 20060101
D01F009/127; D01F 9/133 20060101 D01F009/133 |
Goverment Interests
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED
RESEARCH OR DEVELOPMENT
[0001] This work was supported in part by an award from the United
States government N68335-05-C-0394. The Government of the United
States may have certain rights in this invention.
Claims
1. A continuous process for producing a carbon nanotube reinforced
continuous fiber preform useful in the manufacture of carbon
nanotube reinforced composite articles, comprising steps of: (a)
dispersing a catalyst precursor throughout a continuous fiber
preform; (b) converting the catalyst precursor into catalytic
particles, the catalytic particles dispersed throughout the
continuous fiber preform; (c) continually moving the treated
preform through a growth furnace; (d) contacting the continuous
fiber preform containing the catalytic particles with a hydrocarbon
precursor gas; and wherein vapor grown carbon nanotubes are formed
in situ at the catalytic particles dispersed throughout the
continuous fiber preform to yield a carbon nanotube reinforced
continuous fiber preform.
2. The process of claim 1, wherein the catalyst precursor comprises
a solution of iron, nickel, cobalt, copper, chromium, molybdenum, a
salt or a mixture thereof.
3. The process of claim 2, wherein the solvent for the catalyst
precursor is, acetone, ethanol, isopropanol, hexane, methanol,
water or mixtures thereof.
4. The process of claim 1, wherein the catalyst precursor is iron
acetate, iron nitrate, iron oxalate, nickel acetate, nickel
nitrate, nickel oxalate, cobalt acetate, cobalt nitrate, cobalt
oxalate, or a mixture thereof.
5. The process of claim 1, wherein the catalyst precursor is a
solution of iron (III) nitrate nonahydrate (ferric nitrate) in
ethanol, acetone or an ethanol/acetone mixture.
6. The process of claim 1, wherein the continuous fiber preform
comprises a carbon preform, a ceramic preform, a glass preform, a
quartz preform, a graphite preform, a metal preform or combinations
thereof.
7. The process of claim 6, wherein the preform is a continuous
multi-filament, braid, weave, yarn or tow.
8. The process of claim 1, wherein the catalyst precursor treated
preform is pyrolyzed to form catalytic particles within the preform
in a pyrolysis furnace.
9. The process of claim 8, wherein the pyrolysis further removes
organic content from the preform.
10. The process of claim 8, wherein the pyrolysis furnace includes
and inlet and an outlet and a mechanism for continuously taking up
the continuous preform as it exits the furnace.
11. The process of claim 10, wherein the pyrolysis furnace further
includes a mechanism for continuously belaying the continuous fiber
preform into the pyrolysis furnace and wherein the rate of belaying
and taking up are approximately equal.
12. The process of claim 8, wherein the pyrolysis of the catalyst
precursor takes place at between about 300.degree. C. and
900.degree. C.
13. The process of claim 8, wherein the pyrolysis of the catalyst
precursor takes place in an inert or oxidizing gas atmosphere.
14. The process of claim 13, wherein the pyrolysis of the catalyst
precursor takes place in an argon or nitrogen atmosphere.
15. The process of claim 8, wherein the pyrolysis of the catalyst
precursor takes place from, between 1 second to 30 minutes.
16. The process of claim 15, wherein the pyrolysis of the catalyst
precursor takes place at from about 1 minute to about 15
minutes.
17. The process of claim 8, wherein the pyrolysis of the catalyst
precursor takes place at from about 500.degree. C. to about
600.degree. C.
18. The process of claim 8, wherein the pyrolyzed preform is fed in
to a front-end of a growth furnace with a precursor gas to induce
growth of carbon nanotubes.
19. The process of claim 18, wherein the fed-in preform is taken-up
at a rear-end of the furnace.
20. The process of claim 19, wherein the residence time of the
preform through the growth furnace is approximately between about 1
minute to 1000 minutes.
21. The process of claim 20, wherein the residence time of the
preform through the growth furnace is between about 1 minutes and
120 minutes.
22. The process of claim 18, wherein the pyrolyzed preform has a
heat treatment step prior to induction of nanotube growth.
23. The process of claim 22, wherein the heat treatment step and
the nanotube growth step occur in the same furnace.
24. The process of claim 22, wherein the nanotube growth step
occurs sequentially after the heat treatment step.
25. The process of claim 22, wherein the heat treatment step occurs
at a temperature of about approximately 600.degree. C. to about
900.degree. C.
26. The process of claim 25, wherein the heat treatment step occurs
at a temperature of about 800.degree. C.
27. The process of claim 22, wherein the heat treatment step
happens in an inert atmosphere.
28. The process of claim 18, wherein the nanotube growth step
occurs at a temperature of about approximately 700.degree. C. to
about 950.degree. C.
29. The process of claim 28, wherein the nanotube growth step
occurs at a temperature of about 750.degree. C. to about
850.degree. C.
30. The process of claim 18, wherein the precursor gas has a flow
velocity in the furnace of approximately about 10 to 1000
cm/min.
31. The process of claim 30, wherein the flow velocity in the
furnace is approximately about 100 to 150 cm/min.
32. The process of claim 18, wherein the precursor gas is provided
in a reactive gas composition comprising about approximately 0.1%
to 10% hydrocarbon precursor gas in 99.9% to 90% inert gas.
33. The process of claim 32, wherein the precursor gas is provided
in a reactive gas composition comprising about approximately 0.5%
to 2% hydrocarbon precursor gas in 99.5% to 98% inert gas.
34. The process of claim 33, wherein the precursor gas is provided
in a reactive gas composition comprising about approximately 1%
hydrocarbon precursor gas in 99% inert gas.
35. The process of claim 34, wherein the reactive gas composition
is 1% acetylene in nitrogen.
36. The process of claim 22, wherein the wherein the growth furnace
is a two-zone furnace and heat treatment occurs in a first zone and
nanotube growth occurs in a second zone.
37. The process of claim 36, wherein each zone has a different
temperature.
38. The process of claim 36, wherein the hydrocarbon precursor gas
is entered into the furnace after the heat treatment zone.
39. The process of claim 36, wherein the hydrocarbon precursor is
entered into the furnace before the heat treatment zone but is not
mixed with the purge gas until the second zone.
40. The process according to claim 1 wherein step (b) is carried
out under reducing conditions.
41. The process of claim 1, wherein the hydrocarbon precursor gas
is, acetylene, methane, propane, ethane, ethylene, benzene, natural
gas or mixtures thereof.
42. The process of claims 1, wherein multiple preforms are
processed concurrently.
43. A carbon nanotube reinforced continuous fiber preform produced
by the process of claim 1.
44. The carbon nanotube reinforced continuous fiber preform of
claim 43, wherein the fiber preform is carbon, quartz, glass,
ceramic or metal multi filament yarn, tow, braid or weave.
45. A furnace useful for fabricating a continuous preform having
vapor grown carbon nanotubes grown thereon in a continuous process
comprising: a tube furnace having an inlet and an outlet and a
growth zone; a mechanism for continuously feeding the preform into
the inlet and a mechanism for continuously taking up the preform at
the outlet; and wherein the rate of feeding-in and taking-up are
approximately equal such that the continuous fiber preform is
continuously fed into the furnace for the continuous process of
growing carbon nanotubes, in situ on the continuous preform.
46. The furnace of claim 45, wherein an inert gas purge is applied
to the furnace at the inlet.
47. The furnace of claim 45, wherein the furnace further includes
heat treatment zone.
48. The furnace of claim 47, wherein a hydrocarbon precursor gas is
entered into the furnace after the first zone and before the second
zone.
49. The furnace of claim 47, wherein the hydrocarbon precursor gas
is entered into the furnace before the first zone and mixed with
the purge gas before the growth zone.
50. A process for providing a carbon nanotube reinforced composite
article comprising steps of: (a) dispersing a catalyst precursor
throughout a continuous fiber preform; (b) converting the catalyst
precursor into catalytic particles, the catalytic particles
dispersed throughout the continuous fiber preform; (c) continually
moving the treated preform through a pyrolysis furnace; wherein
vapor grown carbon fibers are deposited in situ at the catalytic
particles throughout the continuous fiber preform to yield a carbon
reinforced continuous fiber preform; and (d) contacting the
continuous fiber preform containing the catalytic particles with a
hydrocarbon precursor gas to yield a carbon reinforced continuous
fiber preform; (e) infusing the carbon reinforced continuous fiber
preform with a thermoplastic or thermoset polymer, thermoplastic or
thermoset polymer resin, metal, ceramic, ceramic precursor, or
amorphous glass to provide a carbon nanotube reinforced composite
article.
51. A carbon nanotube reinforced composite article produced by a
process according to claim 50.
Description
FIELD OF THE INVENTION
[0002] This invention relates to a novel continuous process to
fabricate continuous fiber composites reinforced with vapor grown
carbon nanotubes (VGCNT). In particular, the invention relates to a
continuous process for the production of continuous fiber preforms
useful in the making of carbon nanotube reinforced composite
articles.
BACKGROUND OF THE INVENTION
[0003] Idealized Carbon Nanotubes (CNT) can be visualized as 3-D
graphite sheets rolled to form seamless cylinders, closed with end
caps on both the sides. These end caps have fullerene like
structure. A defect free nanotube has exceptional mechanical,
electrical and thermal properties. The defects can be in the
structure or in the morphology. These structures have stability
greater than that of graphite, thermally as well as chemically.
Based on the diameter of the nanotubes, CNTs are broadly classified
into: (1) Single Wall Carbon Nanotubes; (2) Multi Wall Carbon
Nanotubes; and (3) Carbon Nanofibers.
[0004] Since carbon nanotubes have such extraordinary mechanical
properties, one would expect that, when these are embedded into
some other matrix material, the properties of the matrix would
improve drastically. But, this actually does not happen. This is
because of poor load transfer between the nanotubes and matrix.
Nanotubes are chemically very inert and are not compatible with any
other material in their pristine state. Also, because nanotubes are
typically not well-dispersed, they create stress concentrations and
the high surface area of the nanotubes is compromised. To fabricate
composites with improved properties, it is important to modify the
surface of the nanotubes so as to achieve a good interface and at
the same time, improve the dispersion of the carbon nanotubes in
the matrix. The poor dispersion and poor interfacial interactions
between the nanotubes and the matrix also result in lower than
expected electrical and thermal properties in the resulting
composite material.
[0005] To more optimally fabricate such composites, various
modifications have been proposed. Instead of dispersing the
nanotubes in the matrix and then using the matrix to make composite
structures, an attempt is made to grow the nanotubes on the
composite preforms and then fill the preforms with matrix material.
Using this procedure, the problem of dispersion of nanotubes can be
by-passed.
[0006] The present invention is useful for applications in numerous
industries including composite materials, filtration materials,
electrodes, membranes, cell growth supports, catalysis, and many
other novel and emerging applications that will benefit from this
unique technology. In particular, the present invention relates to
novel non-woven, woven and braided continuous fiber composite
preforms that are subsequently reinforced with vapor grown carbon
fibers that are grown in situ in the preform using a continuous
growth process, the preforms so made and composite articles made
using the preform. The resulting continuous fiber VGCNT reinforced
composite preform exhibits increased fiber volume fraction of
reinforcing fibers and greatly increased surface area thus
improving the strength, stiffness, electrical conductivity, and
thermal conductivity of polymer matrix composites produced from
these preforms while maintaining the manufacturing benefits of a
continuous non-woven, braided or woven preforms. The resulting
articles produced from VGCNT infused preforms produced in this way
are useful for numerous applications that take advantage of the
unique structural, morphological, electrical, and thermal
properties.
[0007] Polymer matrix composites are well known for use in
structural and thermal-structural applications. Continuous yarn,
and other multidimensional 2-D and 3-D, non-woven, woven or
braided, composite preforms are used in the manufacture of
reinforced composites due to their economical manufacturing
processes. For the purposes of this invention the term "preform"
means a continuous fiber yarn, tow, or broad good produced from the
tow or yarn (including non-woven mats, woven or braided
constructions) and assemblies of preforms further constructed.
Through weaving or braiding of the reinforcing fiber yarns of
carbon or graphite (carbon and graphite fibers are generally
referred to collectively as "carbon fiber" and the term "carbon
fiber" is used throughout to mean "carbon and/or graphite fiber"),
glass, quartz, metal or ceramic fiber a composite "preform" can be
manufactured into a near net shape that is subsequently infused
with a polymer resin and cured in a mold to manufacture
articles.
[0008] Preforms may also be infused with a polymer or polymer resin
to manufacture a prepreg useful for the fabrication of polymer
matrix composite material articles. Using well-known methods the
non-woven, woven or braided preforms are manipulated by slitting,
combining together, stitching together, shaping, or other methods
to assemble a near net shape preform for the fabrication of a
composite article. The continuous fiber preform processes are
advantageous since they can be used very economically to produce a
variety of shapes useful in the manufacture of composites. A
fundamental limitation of certain woven, braided, non-woven mat, or
felt preform technology to date, however, has been that the
resulting composites manufactured from these preforms are of lower
strength and stiffness than composites manufactured using other
methods. This is due in part to the lower fiber volume fraction
that results from the weaving and braiding processes and in part to
the failure mechanisms of weave braid or tow composites
intrinsically related to the reinforcing fiber geometry and
architecture.
[0009] A goal in composite materials design has been to obtain
materials which exhibit high stiffness, strength, fracture
toughness, controllable electrical and thermal properties and can
be affordably manufactured. Hence, there exists a need for a novel
approach to improve the stiffness, strength, fracture toughness,
and the electrical and thermal properties, of woven and braided
composite materials while maintaining the low cost advantages of
continuous woven and braided preform manufacturing.
[0010] The present invention is concerned with the use of VGCNT
produced in a composite preform to improve the mechanical,
electrical, and thermal characteristics of composite materials
produced from these preforms as well as novel materials and
articles that can be produced from the preforms themselves. VGCF
and VGCNT are produced directly from hydrocarbons such as methane,
acetylene, methane, propane, ethane, ethylene, benzene, natural gas
or hydrocarbon gas mixture, in a gas phase reaction upon contact
with a catalytic metal particle in a non-oxidizing gas stream.
Various reaction processes, conditions, and chambers are known and
described in e.g., U.S. Pat. Nos. 5,024,818 and 5,374,415 for the
manufacture of VGCF. Vapor grown carbon fibers differ substantially
from commercial carbon fibers in that the VGCF are not continuous.
The VGCF and VGCNT can vary in diameter and length depending on
processing parameters, including catalyst particle characteristics,
reactive gas composition, pyrolysis time and temperature, heat
treatment time and temperature, and length of growth period and
volume of furnace, but exhibit diameters in the range of 1 to 500
nm and lengths in the range of 0.1 .mu.m to 500 .mu.m.
[0011] More importantly and pertinent to this application is that
the fiber diameter of a vapor grown carbon fiber is generally under
1 .mu.m. As those familiar with the growth of vapor grown carbon
fibers know, these fibers can be subsequently thickened to the
diameter of commercial fibers. However, these fibers are not as
desirable from an economic or performance perspective. It is
desirable to use fibers that are smaller than the diameter of a
commercial fiber by a factor in the range of 10 to 100.
[0012] Further, as the vapor grown carbon fibers are much finer
than continuously produced carbon fibers they can be used
effectively to increase the fiber volume fraction of a continuous
fiber composite by occupying the void spaces between the continuous
fibers. The fine diameter vapor grown carbon fibers can occupy
spaces in non-woven, woven or braided composite preforms without
perturbing the geometry, orientation, or continuous fiber volume
fraction of the preform. The result is that the overall fiber
volume fraction is increased leading to desirable changes in the
mechanical, electrical, and thermal behavior of composite materials
manufactured from these preforms.
[0013] A further distinctly novel advantage of this approach is
that vapor grown carbon nanotubes are intimately and uniformly
incorporated into a composite material. The in situ process to
produce the nanotube reinforced preform ensures that nanofibers are
well distributed throughout the preform and are in intimate contact
with themselves and the continuous fiber of the preform.
Conventionally, carbon nanotubes are grown, separated from their
substrate and incorporated into a composite material by mixing
and/or dispersing the nanofibers into the matrix. The
nanofiber/matrix mixture is then used to prepare composites. This
method has many disadvantages including cost, additional process
operations, nanofiber damage from mixing, and negative impact on
matrix rheology. Further, the nanofibers must be handled and
possible health risks from nanofiber exposure is a concern. U.S.
patent application Ser. No. 11/057,462 discloses methods in which
VGCF are produced directly from catalytic particles formed on the
continuous fiber surface, thus the nanofibers are fused to the
continuous fiber surface and act to enhance the adhesion of the
composite matrix to the continuous fiber, further improving the
properties of resulting composite materials. However, the invention
described in Ser. No. 11/057,462 is limited in that the continuous
preform is static in the reaction vessel and is therefore limited
in its length to the size of the vessel and further, the reaction
conditions to which the preform is subjected.
[0014] A further distinctly novel advantage of the present
invention is to change the electrical conductivity of a
non-conducting composite material preform at very low levels of
nanotube. It is known that carbon nanotubes can be incorporated
into a polymer matrix by mixing, blending, solvent-assisted
blending, or other similar techniques. At a certain fraction of
nanotubes, the polymer composites made in this way become
conductive due to continuous contact of the inherently conductive
nanotubes. The point at which this continuous conduction occurs is
commonly referred to as the "percolation threshold." In
conventional nanotube composites where the nanotubes are mixed into
the matrix material this typically occurs at weight fractions of
nanotube to polymer of 1% to 30% depending on the nanotube
morphology, mixing techniques, and other variable factors. In the
novel approach described in this invention conductivity occurs at
nanotube levels approximately ten times lower. This phenomenon is
because the nanotubes are not broken down in aspect ratio and their
intimate contact with each other is not disrupted by mixing and
dispersion processes.
[0015] There are limited literature reports of attempts to produce
catalytically vapor grown carbon nanotubes on graphite, carbon,
quartz, glass or metal substrates. However, the methods differ
substantially from the method described in this invention and none
report continuous in situ production of VGCNT on carbon fiber
yarns, tows, non-woven, woven or braided preforms and are thus
further limited in their utility due to the limited ability to
incorporate such VGCNT into a composite article.
[0016] Hernadi et al. (1996) report on VGCF produced on graphite
flakes using an iron catalyst and acetylene/nitrogen gas mixture.
They treated the graphite flakes with iron acetate and then reduced
under hydrogen at 1200.degree. C. to produce metallic iron
particles. VGCF were subsequently produced at 700.degree. C. in a
flowing acetylene/nitrogen gas at atmospheric pressure. The
reported yield was extremely low at 3.4% with poor quality
nanofibers. Yacaman et al. (1993) also reported VGCF produced on
graphite flakes using an iron catalyst and acetylene/nitrogen gas
mixture. They treated the graphite flakes with an iron oxalate
solution and reduced the catalyst to metallic iron particles under
hydrogen at 350.degree. C. VGCF were subsequently produced at
700.degree. C. in a flowing acetylene/nitrogen gas at atmospheric
pressure for several hours. They reported nanofibers were produced
with diameters in the range of 5.0 to 20 nanometers and lengths of
around 50 micrometers, however, after 1 hour of growth graphitic
structures were noted around few catalytic particles. Ivanov et al.
(1995) reported production of VGCF on graphite flakes using an iron
catalyst and acetylene/nitrogen gas mixture. They treated the
graphite flakes with an iron oxalate solution followed by
calcination at 500.degree. C. followed by reduction with hydrogen
at 500.degree. C. for 8 hours. Under optimal conditions they
reported VGCF with average diameter of 40-100 nanometers and
average length of 50 micrometers and 50% amorphous carbon. Wang et
al. (2002) reported VGCF produced on graphite foil by sputter
coating with stainless steel (Fe:Cr:Ni--70:19:11) followed by
hydrogen reduction at 660C. VGCF were subsequently produced at 0.3
torr pressure using an acetylene/nitrogen mixture. Significantly,
they reported that for a pure iron or nickel catalyst on graphite
no VGCF were formed. Thostenson et al. (2002) used identical
process conditions as Wang et al. (2002) for growth of VGCF on a
carbon fiber. They reported a nanofiber growth layer region between
200-500 nm in thickness.
[0017] U.S. Pat. Nos. 5,165,909 and 6,235,674 to Tennent et al.,
discuss the possibility of producing carbon fibrils, fibril mats,
furry fibers, furry plates, and branched fibrils by deposition of a
metal-containing particle on the surface of a carbon or alumina
fiber, plate, or fibril and subsequent chemical vapor catalytic
growth of carbon fibrils on the substrate at temperatures in the
range of 850.degree. C. to 1200.degree. C. This example requires
deposition of a preformed catalyst particle onto a carbon substrate
and furthermore no working examples were provided other than
branched fibrils. However, the approach Tennent et al., was very
limited because it requires a separate process to form catalytic
particles and disperse them. Such dispersion is not possible with a
multi-filament yarn of continuous macroscopic fiber or a woven or
braided preform manufactured from a multi-filament yarn.
[0018] In all these cases no discussion or method exists for the
production of VGCNT on continuous carbon fiber yarns and preforms
both mono- and multi-filament with sufficient yield in an
industrially practical process. Thostenson et al. (2002) is the
only literature report of VGCF growth on a carbon fiber, but in
that case they used a stainless steel sputter coated fiber and
specifically mentioned that catalyst could only be deposited on the
outermost surfaces of a fiber bundle, not the interior fibers - and
the process required a lengthy hydrogen reduction step to form
catalytic particles. Further, VGCF growth was performed under high
vacuum. None of these process steps are amenable to practical,
scalable, manufacturing of nanofiber reinforced preforms.
[0019] U.S. patent application Ser. No. 11/057,462 (now U.S. Pat.
No. 7,338,684, hereby incorporated in its entirety for all
purposes) describes the fabrication of continuous preform having in
situ grown VGCF. However, while the '462 application describes the
benefits of in situ growth of VGCF on continuous fibers, the
methods provided are limited to "batchwise" processing. The current
invention provides for the in situ growth of VGCNT on continuous
fiber preforms and continuous processing of the continuous fiber
such that the time, type of fiber and characteristics of the VGCNT
can be tailored to the demands of the preform or ultimate composite
made therefrom. This eliminates the processing steps for isolated
carbon nanotubes reported in other carbon nanotube composite
approaches and therefore greatly reduces risk of environmental
release and exposure to carbon nanotubes. A further limitation of
the art is that to be usable, the VGCNT laden preforms need to be
made in large volumes such that there use is not limited to short
pieces or sections able to fit in a single reaction vessel.
SUMMARY OF THE INVENTION
[0020] The present invention provides a continuous process for the
fabrication of vapor grown carbon nanotubes on a continuous
preform. The continuous preform thus made according to the
invention, is not limited in size or length and its progress
through and exit from the growth furnace can be controlled. The
continuous preform thus fabricated with VGCNT grown in situ
demonstrate improved stiffness, strength, fracture toughness, and
tailorable electrical and thermal properties in composite articles
manufactured therefrom. It will be appreciated by those of skill,
that the preforms thus made are essentially endless, not being
limited in their capacity to be formed into composites by length or
size. In another aspect, the present invention provides a
continuous method for manufacturing in situ of a vapor grown carbon
nanotube reinforced composite preform useful in many industrial
applications. In yet another aspect, the present invention provides
a method for the manufacture of composite articles from these
continuously grown VGCNT reinforced composite preforms. Thus, the
present invention provides a continuous process for growing VGCNT
in situ in a continuous preform at treatment periods and growth
times that are tailored to each specific substrate or preform type
and for the growth of VGCNT having desired characteristics.
[0021] Therefore, in one exemplary embodiment, the invention
comprises a continuous process for producing a carbon nanotube
reinforced continuous fiber preform useful in the manufacture of
carbon reinforced composite articles. This exemplary embodiment
comprises the steps of: (a) dispersing a catalyst precursor
throughout a continuous fiber preform; (b) converting the catalyst
precursor into catalytic particles, the catalytic particles
dispersed throughout the continuous fiber preform; (c) contacting
the continuous fiber preform containing the catalytic particles
with a hydrocarbon precursor gas; and (d) continually moving the
treated preform through a growth furnace. Using this process vapor
grown carbon nanotubes are formed in situ at the catalytic
particles dispersed throughout the continuous fiber preform to
yield a carbon nanotube reinforced continuous fiber preform.
[0022] In various exemplary embodiments according to the invention,
the catalyst precursor comprises iron, nickel, cobalt, copper,
chromium, molybdenum, or a mixture thereof or any usable salt
thereof Of course it should be appreciated that the invention is
not limited to the particular catalysts recited above, as any
catalyst suitable for VGCNT is encompassed by the invention. In
some exemplary embodiments, the solvent for the catalyst precursor
is alcohol, acetone, ethanol, isopropanol, hexane, methanol, water
or any other suitable solvent usable for the catalyst precursor. In
some exemplary embodiments, the catalyst precursor is a salt.
Further, it should be appreciated that in some instances, the
solvent will be a mixture of any of the foregoing. In still other
exemplary embodiments, the catalyst precursor is iron acetate, iron
nitrate, iron oxalate, nickel acetate, nickel nitrate, nickel
oxalate, cobalt acetate, cobalt nitrate, cobalt oxalate, or a
mixture thereof Those of skill in the art however, will recognize
that any other catalyst precursor suitable for growing VGCNT will
do.
[0023] In various exemplary embodiments, the continuous fiber
preform includes a carbon preform, ceramic preform, glass preform,
quartz preform, a graphite preform, a metal preform or combinations
thereof For example, it will be appreciated by those of skill in
the art, that a preform of one type may be joined to a preform of
another type for use in the continuous process disclosed herein. In
some exemplary embodiments, the continuous fiber is a
multi-filament fiber. In this embodiment, the multi filament fiber
may include a yarn, a weave, a braid or a tow. Of course, it should
be appreciated that the multifilament fiber according to the
invention may be combinations of the above such that the continuous
fiber is joined to another continuous fiber of the same or
different type as desired.
[0024] In various other exemplary embodiments, the invention
includes pyrolysis of the catalyst treated preform in a pyrolysis
furnace. Those of skill in the art will appreciate that pyrolysis
of the catalyst precursor treated preform burns off organic
material in the continuous fiber preform and converts the catalyst
precursor molecules to catalytic particles. In some exemplary
embodiments according to the invention, the process of pyrolyzing
the continuous preform is a continuous process. In this exemplary
embodiment, the process includes a pyrolysis furnace adapted for
the continuous deployment of the continuous fiber preform through
the pyrolysis furnace. In various exemplary embodiments according
to the invention, the pyrolysis furnace is provided with a
mechanism for continuously loading the continuous fiber preform
into an inlet of the pyrolysis furnace and continuously taking-up
the continuous fiber at an outlet of the pyrolysis furnace. In this
embodiment the rate of loading and of taking up is approximately
equal such that the continuous fiber preform moves through the
furnace at a predetermined, rate. Of course, it should be
appreciated that the rate of transit through the pyrolysis furnace
can vary depending on the temperature, volume of the furnace,
composition of the preform etc.
[0025] In various exemplary embodiments, the pyrolysis of the
continuous fiber preform takes place at between about 300.degree.
C. and 900.degree. C. In some exemplary embodiments, the pyrolysis
of the continuous fiber preform takes place at between about
500.degree. C. to about 600.degree. C. In various exemplary
embodiments, the pyrolysis of the preform takes from between one
second to over thirty minutes. Those of skill in the art will
appreciate that the time for pyrolysis is a function of the
temperature at which the pyrolysis takes place and the residence
time in the furnace. If the pyrolysis furnace is hotter pyrolysis
will take less time. In various exemplary embodiments, the
pyrolysis takes place at from about one minute to about 15
minutes.
[0026] In various exemplary embodiments, the process according to
the invention further includes feeding the pyrolyzed continuous
preform into the front-end of a growth furnace with a precursor gas
to induce growth of carbon nanotubes. In some exemplary embodiments
according to the invention, the fed-in preform is taken-up at the
rear-end of the growth furnace. In various exemplary embodiments,
the growth furnace includes a mechanism at the front-end and the
rear-end such that the continuous fiber preform moves at a
continuous, predetermined rate through the growth furnace. In
various exemplary embodiments, the time of passage through the
growth furnace is from 1 minute to 1,000 minutes. In some exemplary
embodiments, the residence time in the growth furnace is from about
10 minutes to about 100 minutes. However, those of skill in the art
will appreciate that the residence time in the furnace is a factor
of the temperature of the furnace, the length of the furnace and
the length of the preform. Thus, while in some exemplary
embodiments, the temperature of the growth furnace is about from
between approximately 700.degree. C. to about 950.degree. C., in
other exemplary embodiments the nanotube growth step occurs at a
temperature of about 750.degree. C. to about 850.degree. C.
[0027] In various exemplary embodiments, the hydrocarbon precursor
gas is acetylene, methane, propane, ethylene, benzene, natural gas
or mixtures thereof. Of course it should be appreciated that any
suitable hydrocarbon precursor gas is encompassed by the invention.
In some exemplary embodiments, the hydrocarbon precursor gas is
provided in a reactive gas composition comprising about
approximately 0.1% to 10% hydrocarbon precursor gas in 99.9% to 90%
inert gas. In various exemplary embodiments, the precursor gas is
provided in a reactive gas composition comprising about
approximately 0.5% to 2% hydrocarbon precursor gas in 99.5% to 98%
inert gas. In some exemplary embodiments, the precursor gas is
provided in a reactive gas composition comprising about
approximately 1% hydrocarbon precursor gas in 99% inert gas. In
some exemplary embodiments, the reactive gas composition is 1%
acetylene in nitrogen. In various embodiments, the flow velocity in
the furnace is approximately about 10 to 1000 cm/min. In still
other exemplary embodiments, flow velocity in the furnace is
approximately about 10 to 100 cm/min. Of course, those of skill in
the art will appreciate that the velocity of flow in the furnace
will be a function of the size of the furnace and the length of the
furnace, e.g., volume. Thus, the residence time of the continuous
fiber preform in the furnace will be a function of many factors
each optimized for the particular fiber, furnace gas mixture etc.
used.
[0028] In various other exemplary embodiments, the continuous fiber
preform has a heat treatment step before the growth step. In this
exemplary embodiment, the heat treatment step occurs at a
temperature of about approximately 600.degree. C. to about
900.degree. C. In some exemplary embodiments, the heat treatment
step occurs at a temperature of about 800.degree. C. In various
exemplary embodiments, the heat treatment step occurs in an inert
atmosphere.
[0029] In various exemplary embodiments, the heat treatment step
occurs sequentially before the growth step. In some exemplary
embodiments, the heat treatment step and the growth step take place
in the same furnace. In these exemplary embodiments, the furnace is
a two-zone furnace such that the continuous fiber preform moves
from the heat treatment zone to the growth zone without exiting the
furnace. In some exemplary embodiments, the inert gas (or purge
gas) is added to the furnace at the inlet and the hydrocarbon
precursor gas is injected into the furnace before the growth zone.
In other exemplary embodiments, the purge gas is added to the
furnace at the inlet and the hydrocarbon precursor gas is entered
to the furnace via a second inlet, prior to the heat treatment
zone. In this embodiment, the hydrocarbon precursor gas is mixed
with the purge gas prior to the growth zone. In various exemplary
embodiments, the continuous fiber preform is continuously moved
through the furnace at a predetermined rate. In some exemplary
embodiments, the furnace includes a mechanism at the inlet and the
outlet such that the continuous fiber preform is belayed into the
furnace and taken-up at the furnace outlet and approximately equal
rates, such that the continuous fiber preform continuously moves
through the heat treatment zone and the growth zone allowing for
the continuous growth of VGCNT along the length of the continuous
fiber preform. Of course, those of skill in the art will recognize
that the rate of movement of the continuous fiber preform through
the furnace can be slower or faster, the rate of gas flow and
temperature adjusted thereto depending on the VGCNT growth desired,
the specific hydrocarbon precursor used and the length of the
furnace, or the specific type of continuous preform used, to name a
few of the variables. Further, it should be appreciated that more
than one continuous fiber preform can be processed at one time.
Further, according to some embodiments, the continuous fiber
preforms of the same type or of a different type are treated with
the catalyst precursor in tandem, pyrolyzed and fed into the growth
furnace. In other embodiments, one or more continuous fiber
preforms have already been treated and have been stored for later
use. These preforms may then be fed into the growth furnace
together to allow for VGCNT growth on more than one preform during
the growth phase.
[0030] Thus, it should be appreciated that using the disclosed
methods, any of these parameters can be changed as desired. For
example, the reactive gas composition, preform type, residence time
etc. can be altered as the preform moves through the furnace as
desired.
[0031] In various other exemplary embodiments, the invention
includes a process of fabricating a carbon nanotube reinforced
composite article using the foregoing process and further infusing
the carbon reinforced continuous fiber preform with a thermoplastic
or thermoset polymer resin, metal, ceramic, ceramic precursor, or
amorphous glass to provide a carbon nanotube reinforced composite
article.
[0032] In still other exemplary embodiments the invention includes
a continuous fiber preform made by a continuous process as
described above. The inventors have found that the VGCNT fibers
formed by the instantly disclosed process are denser, more uniform,
well dispersed and in intimate contact, and provide more structural
support and electrical conductivity because they are grown in
situ.
[0033] In yet other exemplary embodiments, the invention includes a
carbon nanotube reinforced composite article produced from the
infusing the continuous fiber preform disclosed herein using a
desired resin or matrix.
[0034] Other objects, features and advantages of the present
invention will become apparent after review of the detailed
description, figures and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1A is an SEM image of carbon preform sample from
EXAMPLE 1 (approx. 3950.times. magnification).
[0036] FIG. 1B is an SEM image of carbon preform sample from
EXAMPLE 1 (approx. 23,600.times. magnification).
[0037] FIG. 2 is an SEM image of carbon preform sample from EXAMPLE
2 (approx. 3950.times. magnification).
[0038] FIG. 3 is an SEM image of carbon preform sample from EXAMPLE
3 (approx. 3950.times. magnification).
[0039] FIG. 4 is an SEM image of carbon preform sample from EXAMPLE
4 (approx. 3950.times. magnification).
[0040] FIG. 5 is an SEM image of carbon preform sample from EXAMPLE
5 (approx. 23,600.times. magnification).
[0041] FIG. 6 is a diagrammatic representation of the first step in
the process for the fabrication of a continuous preform in one
embodiment according to the invention. Step 1, is the preform
catalyst solution treatment process. This in general involves
solution coating and drying the preform.
[0042] FIG. 7 is a diagrammatic representation of the second step
in the process for the fabrication of a continuous preform in one
embodiment according to the invention. Step 2, is the preform
catalyst pyrolysis process. This in general involves heating the
dried preform in an inert atmosphere to pyrolyze the organic
content of sizings and to convert the catalyst precursor to
catalytic particles.
[0043] FIG. 8 is a diagrammatic representation of Phase I of steps
3 and 4 in the process for the fabrication of a continuous preform
according to one exemplary embodiment of the invention. This figure
illustrates the continuous catalyst heat-treatment and nanotube
growth process using a two-part tube furnace.
[0044] FIG. 9 is a diagrammatic representation of Phase II of steps
3 and 4 in the process for the fabrication of a continuous preform
according to one exemplary embodiment of the invention. This figure
illustrates the continuous catalyst heat-treatment and nanotube
growth process using a unitary tube furnace with an intervening
baffle.
[0045] FIGS. 10A and 10B are electron micrographs of carbon
nanotubes (VGCNT) grown on silicon carbide (SiC) fibers
(Nicalon.TM., Nippon Chemical LTD, Japan) fiber. FIG. 10A, shown at
about 620.times., a segment of the SiC fiber yarn with 14 minutes
of heat treatment and 42 minutes of growth. FIG. 10B, shown at
about 540.times., segment of the SiC fiber yarn with 6 minutes of
heat treatment and 50 minutes of growth.
[0046] FIGS. 11A and 11B are electron micrographs of CNT grown on
un-sized Hexcel Corporation AU4-12k carbon fiber yarn. FIG. 11A,
shown at about 870.times., shows a segment of yarn 45 cm from the
leading edge of the catalyst treated continuous yarn corresponding
to a heat treatment time of 14 minutes and a growth time of 42
minutes. FIG. 11B, shown at about 650.times., shows a segment of
yarn 51 cm from the leading edge corresponding to a heat treatment
time of 10 minutes and a growth time of 46 minutes.
[0047] FIG. 12, shown at 160.times. magnification is an electron
micrograph showing CNT Carbon nanotube growth on a segment of SiC
(Hi-Nicalon.TM.) fiber yarn from approximately the middle of the 30
m length of yarn.
[0048] FIGS. 13A and 13B are scanning electron micrographs of
carbon nanotube growth on a 15 cm piece of ceramic grade SiC fiber
yarn (Nicalon.TM. CG) demonstrating lower electrical resistivity.
FIGS. 13A and B are the same preparation, at 350.times. SEM, and
1300.times. magnification respectively.
[0049] FIG. 14 is a graph comparing the load-displacement curves
for unidirectional composites fabricated with conventional
(control) and VGCNT-treated ceramic grade SiC fiber yarn
(Nicalon.TM. CG) according to one embodiment of the invention. As
shown, the control product is less stiff and upon application of a
load the control failed while the composite according to the
invention illustrates over 200% greater elongation to failure and
55% higher fracture toughness.
[0050] FIGS. 15A and 15B are photographs showing uni-directional
ceramic matrix composite green body using VGCNT treated SiC yarn
FIG. 15A and a melt-infiltrated ceramic matrix composite article
fabricated there from FIG. 15B.
[0051] FIGS. 16A and 16B are electron micrographs of carbon
nanotubes on a 5 m piece of SiC (Hi-Nicalon.TM.) yarn using an
Fe(NO.sub.3).sub.3.9H.sub.2O/IPA catalyst solution followed by
dipping in hexanes and subsequent cleaning by nitric acid reflux
after growth. FIG. 16A shows a segment of the preparation at
950.times. magnification. FIG. 16B, the same preparation at a
2,400.times. magnification.
[0052] FIGS. 17A and 17B are electron micrographs showing VGCNT
growth obtained on SiC (Nicalon.TM.) yarns using cobalt acetate
catalyst. FIG. 17A is an electron micrograph of carbon nanotubes on
SiC (Nicalon.TM.) yarn at a 30.times. magnification. FIG. 17B shows
the same preparation at 1410.times. magnification.
[0053] FIGS. 18A and 18B are SEM images showing VGCNT grown on SiC
fiber (Hi-Nicalon.TM.). FIG. 18A shows a segment of the SiC with 92
minutes of growth at 540.times. magnification. FIG. 18B shows the
same preparation at 750.times. magnification.
DETAILED DESCRIPTION OF THE INVENTION
[0054] I. In General
[0055] It must be noted that as used herein and in the appended
claims, the singular forms "a", "an", and "the" include plural
reference unless the context clearly dictates otherwise. As well,
the terms "a" (or "an"), "one or more" and "at least one" can be
used interchangeably herein. It is also to be noted that the terms
"comprising", "including", "characterized by" and "having" can be
used interchangeably.
[0056] Unless defined otherwise, all technical and scientific terms
used herein have the same meanings as commonly understood by one of
ordinary skill in the art to which this invention belongs. All
publications and patents specifically mentioned herein are
incorporated by reference for all purposes including describing and
disclosing the chemicals, instruments, statistical analyses and
methodologies which are reported in the publications which might be
used in connection with the invention. All references cited in this
specification are to be taken as indicative of the level of skill
in the art. Nothing herein is to be construed as an admission that
the invention is not entitled to antedate such disclosure by virtue
of prior invention. It should be appreciated that the term
"preform" according to the invention is the "substrate" for the
VGCNT grown in situ thereon. Further, as used herein the terms
"hydrocarbon precursor gas" and "feed gas" are used interchangeably
and are equivalents. Further, it should be appreciated that, as
used herein, the term "endless" refers to the ability to bond,
braid, weave or combine in any like manner multiple continuous
carbon fiber yarns sequentially, in parallel or any other manner
such that the length or area is essentially infinite or endless. As
used herein the term "reactive gas composition" refers the gas
mixture in the growth chamber. Thus, the reactive gas composition
refers to the hydrocarbon precursor gas in addition to the carrier
or "purge" or inert gas. As is described below, in some instance,
the reactive gas composition is premixed before being fed into the
furnace in other embodiments the reactive gas composition is mixed
within the furnace.
[0057] The invention and the various features and advantageous
details thereof are explained more fully with reference to the
non-limiting embodiments that are illustrated in the accompanying
drawings and detailed in the following description. Descriptions of
well known components and processing techniques are omitted so as
not to unnecessarily obscure the invention in detail but such
descriptions are, nonetheless, included in disclosure by
incorporation by reference of the citations following the Examples
section. It should be understood, however, that the detailed
description and the specific examples, while indicating preferred
embodiments of the invention, are given by way of illustration only
and not by way of limitation. Various substitutions, modifications,
additions and/or rearrangements within the spirit and/or scope of
the underlying inventive concept will become apparent to those
skilled in the art from this detailed description.
[0058] II. The Invention
[0059] In one aspect, the present invention provides a continuous
process for the fabrication of vapor grown carbon fibers on a
continuous preform. The continuous preform thus fabricated with
VGCNT grown in situ demonstrates improved stiffness, strength,
fracture toughness, and tailorable electrical and thermal
properties in composite articles manufactured therefrom. In another
aspect, the present invention provides a continuous method for
manufacturing in situ of a vapor grown carbon nanotube reinforced
composite preform useful in many industrial applications. Such
applications include, but are not limited to, electrodes,
intelligent textiles, electromagnetic signature control,
electromagnetic interference (EMI) shielding, carbon-carbon
precursor material, filtration and separation, thermal management
materials, ceramic composite materials, gas adsorption and solid
state storage, cell growth and tissue regeneration. In yet another
aspect, the present invention provides a method for the manufacture
of composite articles from these continuously grown VGCNT
reinforced composite preforms. Thus, the present invention provides
a continuous process for growing VGCNT in situ in a continuous
preform at treatment periods and growth times that are tailored to
each specific substrate or preform type and for the growth of VGCNT
having desired characteristics.
[0060] Therefore, in one exemplary embodiment, the invention
comprises a continuous process for producing a carbon nanotube
reinforced continuous fiber preform useful in the manufacture of
carbon reinforced composite articles. This exemplary embodiment
comprises the steps of: (a) dispersing a catalyst precursor
throughout a continuous fiber preform; (b) converting the catalyst
precursor into catalytic particles, the catalytic particles
dispersed throughout the continuous fiber preform; (c) contacting
the continuous fiber preform containing the catalytic particles
with a hydrocarbon precursor gas; and (d) continually moving the
treated preform through a growth furnace. Using this process vapor
grown carbon nanotubes are formed in situ at the catalytic
particles dispersed throughout the continuous fiber preform to
yield a carbon nanotube reinforced continuous fiber preform.
[0061] In various exemplary embodiments according to the invention,
the catalyst precursor comprises iron, nickel, cobalt, copper,
chromium, molybdenum, or a mixture thereof or any usable salt
thereof. Of course it should be appreciated that the invention is
not limited to the particular catalysts recited above, as any
catalyst suitable for VGCNT is encompassed by the invention. In
some exemplary embodiments, the solvent for the catalyst precursor
is alcohol, acetone, ethanol, isopropanol, hexane, methanol, water
or any other suitable catalyst usable for the catalyst precursor.
In some exemplary embodiments, the catalyst precursor is a salt.
Further, it should be appreciated that in some instances, the
solvent will be a mixture of any of the foregoing. In still other
exemplary embodiments, the catalyst precursor is iron acetate, iron
nitrate, iron oxalate, nickel acetate, nickel nitrate, nickel
oxalate, cobalt acetate, cobalt nitrate, cobalt oxalate, or a
mixture thereof Those of skill in the art however, will recognize
that any other catalyst precursor suitable for growing VGCNT will
do.
[0062] In various exemplary embodiments, the continuous fiber
preform includes a carbon preform, ceramic preform, glass preform,
quartz preform, a graphite preform, a metal preform or combinations
thereof For example, it will be appreciated by those of skill in
the art, that a preform of one type may be joined to a preform of
another type for use in the continuous process disclosed herein. In
some exemplary embodiments, the continuous fiber is a
multi-filament fiber. In this embodiment, the multi-filament fiber
may include a yarn, a weave, a braid or a tow. Of course it should
be appreciated that the multi-filament fiber according to the
invention may be combinations of the above such that the continuous
fiber is joined to another continuous fiber of the same or
different type as desired. Further, as disclosed herein, in various
exemplary embodiments, more than one continuous fiber can be
treated at one time. Thus, according to some embodiments, the
continuous fiber preforms of the same type or of a different type
are treated with the catalyst precursor in tandem, pyrolyzed and
fed into the growth furnace. In other embodiments, one or more
continuous fiber preforms have already been treated and have been
stored for later use. These preforms may then be fed into the
growth furnace together to allow for VGCNT growth on more than one
preform during the growth phase.
[0063] In various other exemplary embodiments, the invention
includes pyrolysis of the catalyst treated preform in a pyrolysis
furnace. Those of skill in the art will appreciate that pyrolysis
of the catalyst precursor treated preform burns off organic
material in the continuous fiber preform and converts the catalyst
precursor molecules to catalytic particles. In some exemplary
embodiments according to the invention, the process of pyrolyzing
the continuous preform is a continuous process. In this exemplary
embodiment, the process includes a pyrolysis furnace adapted for
the continuous deployment of the continuous fiber preform through
the pyrolysis furnace. In various exemplary embodiments according
to the invention, the pyrolysis furnace is provided with a
mechanism for continuously loading the continuous fiber preform
into an inlet of the pyrolysis furnace and continuously taking-up
the continuous fiber at an outlet of the pyrolysis furnace. In this
embodiment the rate of loading and of taking up is approximately
equal such that the continuous fiber preform moves through the
furnace at a predetermined, rate. Of course, it should be
appreciated that the rate of transit through the pyrolysis furnace
can vary depending on the temperature, volume of the furnace,
composition of the preform etc.
[0064] In various exemplary embodiments, the pyrolysis of the
continuous fiber preform takes place at between about 300.degree.
C. and 900.degree. C. In some exemplary embodiments, the pyrolysis
of the continuous fiber preform takes place at between about
500.degree. C. to about 600.degree. C. In various exemplary
embodiments, the pyrolysis of the preform takes from between one
second to over thirty minutes. Those of skill in the art will
appreciate that the time for pyrolysis is a function of the
temperature at which the pyrolysis takes place and the residence
time in the furnace. If the pyrolysis furnace is hotter pyrolysis
will take less time. In various exemplary embodiments, the
pyrolysis takes place at from about one minute to about 15
minutes.
[0065] In various exemplary embodiments, the process according to
the invention further includes feeding the pyrolyzed preform into
the front-end of a growth furnace with a precursor gas to induce
growth of carbon nanotubes. In some exemplary embodiments according
to the invention, the fed-in preform is taken-up at the rear-end of
the growth furnace. In various exemplary embodiments, the growth
furnace includes a mechanism at the front-end and the rear-end such
that the continuous fiber preform moves at a continuous,
predetermined rate through the growth furnace. In various exemplary
embodiments, the time of passage through the growth furnace is from
1 minute to 1,000 minutes. In some exemplary embodiments, the
residence time in the growth furnace is from about 10 minutes to
about 100 minutes. However, those of skill in the art will
appreciate that the residence time in the furnace is a factor of
the temperature of the furnace, the length of the furnace and the
length of the preform. Thus, while in some exemplary embodiments,
the temperature of the growth furnace is about from between
approximately 700.degree. C. to about 950.degree. C., in other
exemplary embodiments the nanotube growth step occurs at a
temperature of about 750.degree. C. to about 850.degree. C.
[0066] In various exemplary embodiments, the hydrocarbon precursor
gas is acetylene, methane, propane, ethylene, benzene, natural gas
or mixtures thereof. Of course it should be appreciated that any
suitable hydrocarbon precursor gas is encompassed by the invention.
In some exemplary embodiments, the hydrocarbon precursor gas is
provided in a reactive gas composition comprising about
approximately 0.1% to 10% hydrocarbon precursor gas in 99.9% to 90%
inert gas. In various exemplary embodiments, the precursor gas is
provided in a reactive gas composition comprising about
approximately 0.5% to 2% hydrocarbon precursor gas in 99.5% to 98%
inert gas. In some exemplary embodiments, the precursor gas is
provided in a reactive gas composition comprising about
approximately 1% hydrocarbon precursor gas in 99% inert gas. In
some exemplary embodiments, the reactive gas composition is 1%
acetylene in nitrogen. In various embodiments, the flow velocity in
the furnace is approximately about 10 to 1000 cm/min. In still
other exemplary embodiments, flow velocity in the furnace is
approximately about 10 to 100 cm/min. Of course, those of skill in
the art will appreciate that the velocity of flow in the furnace
will be a function of the size of the furnace and the length of the
furnace, e.g., volume. Thus, the residence time of the continuous
fiber preform in the furnace will be a function of many factors
each optimized for the particular fiber, furnace gas mixture etc.,
used
[0067] In various other exemplary embodiments, the continuous fiber
preform has a heat treatment step before the growth step. In this
exemplary embodiment, the heat treatment step occurs at a
temperature of about approximately 600.degree. C. to about
900.degree. C. In some exemplary embodiments, the heat treatment
step occurs at a temperature of about 800.degree. C. In various
exemplary embodiments, the heat treatment step occurs in an inert
atmosphere.
[0068] In various exemplary embodiments, the heat treatment step
occurs sequentially before the growth step. In some exemplary
embodiments, the heat treatment step and the growth step take place
in the same furnace. In these exemplary embodiments, the furnace is
a two-zone furnace such that the continuous fiber preform moves
from the heat treatment zone to the growth zone without exiting the
furnace. In some exemplary embodiments, the inert gas (or purge
gas) is added to the furnace at the inlet and the hydrocarbon
precursor gas is injected into the furnace before the growth zone.
In other exemplary embodiments, the purge gas is added to the
furnace at the inlet and the hydrocarbon precursor gas is entered
to the furnace via a second inlet, prior to the growth zone. In
this embodiment, the hydrocarbon precursor gas is mixed with the
purge gas prior to the growth zone. In various exemplary
embodiments, the continuous fiber preform is continuously moved
through the furnace at a predetermined rate. In some exemplary
embodiments, the furnace includes a mechanism at the inlet and the
outlet such that the continuous fiber preform is belayed into the
furnace and taken-up at the furnace outlet and approximately equal
rates, such that the continuous fiber preform continuously moves
through the heat treatment zone and the growth zone allowing for
the continuous growth of VGCNT along the length of the continuous
fiber preform. Of course, those of skill in the art will recognize
that the rate of movement of the continuous fiber preform through
the furnace can be slower or faster, the rate of gas flow and
temperature adjusted thereto depending on the desires, the specific
hydrocarbon precursor used and the length of the furnace, or the
specific type of continuous preform used, to name a few of the
variables.
[0069] Thus, it should be appreciated that using the disclosed
methods, any of these parameters can be changed as desired. For
example, the reactive gas composition, preform type, residence time
etc. can be altered as the preform moves through the furnace as
desired.
[0070] In various other exemplary embodiments, the invention
includes a process of fabricating a carbon nanotube reinforced
composite article using the foregoing process and further infusing
the carbon nanotube reinforced continuous fiber preform with a
thermoplastic or thermoset polymer resin, metal, ceramic, ceramic
precursor, or amorphous glass to provide a carbon reinforced
composite article.
[0071] In still other exemplary embodiments the invention includes
a continuous fiber preform made by a continuous process as
described above. The inventors have found that the VGCNT fibers
formed by the instantly disclosed process are denser, more uniform
and provide more structural support and electrical conductivity
because they are grown in situ.
[0072] In yet other exemplary embodiments, the invention includes a
carbon nanotube reinforced composite article produced from the
infusing the continuous fiber preform disclosed herein using a
desired resin or matrix.
[0073] In yet other exemplary embodiments, the invention includes a
carbon nanotube reinforced composite article produced from the
infusing the continuous fiber preform disclosed herein.
[0074] Many methods are available for the synthesis of carbon
nanotubes (CNT) such as arc-discharge method, laser ablation method
and chemical vapor deposition (CVD). Amongst these methods, CVD is
the most promising method as the percentage of yield is much higher
and is relatively easy to set-up. This process has the potential to
be scaled up for bulk and continuous production.
[0075] The basic principle of growth in CVD can be explained as
follows. When metal suitable catalysts are saturated with carbon
precursors at high temperatures, CNT are formed. The heated feed
gas decomposes into hydrogen and carbon, with the carbon depositing
on the catalyst particle forming carbon nanotubes. The diameter,
morphology and length of the nanotubes are controlled by varying
the metal catalyst, precursor gases, temperature, reaction time and
substrate (where applicable). In a typical CVD synthesis setup, a
certain amount of catalyst powder/substrate is placed in a quartz
reactor tube and subjected to growth conditions. Generally, a
carrier gas which is an inert gas such as nitrogen, argon or helium
is used (higher flow rate as compared to precursor gas) along with
the carbon feed gas to transport the carbon precursor gas to the
catalyst. The most obvious disadvantage of CVD is the production of
pyrolytic/amorphous carbon along with CNT. While a thin layer of
amorphous carbon on the CNT can be advantageous in certain
applications, a very thick layer of amorphous carbon covering all
the tubes and substrate is certainly detrimental. If the growth
parameters are not controlled carefully, often only amorphous
carbon is deposited. The following table gives the conventional
gases and temperature ranges used for synthesizing each type of
nanotube.
TABLE-US-00001 Type of Carbon Nanotube Temperature (CNT) Precursor
Gas Range Single Wall Carbon Methane Greater than NanoTubes (SWCNT)
900.degree. C. Multi Wall Carbon Acetylene, Ethylene Around
NanoTubes (MWCNT) 750.degree. C. Carbon Nanofibers (CNF) Acetylene,
Ethylene - Between preferably in a plasma 400 and discharge
chamber. 700.degree. C.
[0076] The growth of SWCNT requires greater control of growth
conditions as compared to the growth of MWCNT and CNF. Many
research groups have grown MWCNT on silica and silicon substrates,
glass, graphite flakes etc. Therefore, the inventors have focused
their attention on the growth of a dense crop of CNT on continuous
fiber preforms to yield essentially endless CNT preforms which can
be used to manufacture novel articles and continuous fiber
reinforced composites
[0077] Traditionally, CVD has been used to grow CNT on some
substrate or for bulk growth of CNT using some supported catalyst
powder. The CNT are then harvested for use. The present invention
provides VGCNT grown in situ on the preform substrate, a use for
which the bulk catalyst method is not amenable. In the bulk
synthesis methods, a "floating catalyst" method is used wherein
unsupported catalytic particles float in a fluidized reactor from
which CNTs grow, or metal catalyst is supported by some substrate
which is in the form of fine powder and after the growth; the
substrate is separated to provide free CNT or MWCNT for
incorporation in polymeric materials.
[0078] It should be noted that this is an emerging field.
Therefore, as research into the production and incorporation of CNT
evolves more variables, including substrates and production
variables are recognized. Currently, there are several recognized
variables involved in the synthesis of CNT and new variables arise,
depending on the experimental setup used. Several theories have
been postulated on the growth of CNT and there are two theories
that are widely acceptable. The growth of CNT from a metal catalyst
can either be because of tip growth or base growth. In tip growth,
the catalyst particle stays on top of the CNT and the carbon
deposition takes by passing through the catalyst particle; whereas
in base growth, the catalyst particle adheres to the substrate and
the growth of fiber takes place on the particle. The growth
mechanism therefore depends on the interaction between the catalyst
particle and the substrate; if the catalyst and substrate adhere
weakly, it is likely that the growth takes place via tip growth or
it is base growth if the interaction is strong.
[0079] The growth of CNT and also their morphology is primarily
dependent on the following factors: (1) Substrate; (2)
Catalyst--type of catalyst, size of catalyst particle, amount of
catalyst, method of application of catalyst and pretreatment of
catalyst; (3) Growth temperature; (4) Pressure in the growth
chamber; (5) Carbon precursor gas--type of gas and flow rate; (6)
Duration of growth; and (7) Miscellaneous.
[0080] Substrate
[0081] Many groups have grown CNT on substrates, generally on
silica, zeolite and graphite flakes. It has been concluded that
graphite is not a very good substrate for the growth of CNT using
common catalyst application techniques such as impregnation and
ion-exchange. It was found that for a graphite supported catalyst
the number of catalyst particles encapsulated in amorphous carbon
was higher as compared to a silica gel supported catalyst. It was
found that the growth on other substrates was much better in terms
of yield; this was attributed to the weak interface between the
catalyst particle and graphite. Hence there are no papers that
discuss the growth of CNT on graphite substrate (applying catalyst
by impregnation etc.) by varying the above mentioned parameters. In
the recent past, CNT have been grown on carbon fibers and textiles
by sputter coating them with stainless steel and carrying on the
growth process.
[0082] Catalyst
[0083] Type of Catalyst: Typically, transition metal elements such
as Fe, Co, Ni, Cu, or alloys of these elements are used as
catalysts for the growth of CNT. It is believed that the elements
in their metallic form nucleate the growth of CNT during CVD. In
few experiments, researchers have found that the metallic elements
are completely replaced by some form of carbon after growth.
[0084] Method of application of catalyst: The catalyst can be
applied directly in the metallic state by sputter coating the
substrate with a thin film of metal or it can be used in the form
of metal salt solution. The later method is widely used and
catalyst in the form of aqueous/alcoholic solutions of Iron(III)
Nitrate, Iron (III) Acetate, Iron(III) Oxalate, Nickel(II) Acetate,
Co(II) Acetate, Co(II) Nitrate etc. are applied through
impregnation or ion-exchange. It has been found that Cobalt and
Iron catalyze the formation of well defined hollow structures,
whereas on Nickel and Copper fragments of turbostratic graphite are
formed.
[0085] It should be noted that, each method of application further
presents various variables and drawbacks depending on the desired
use. For example, the impregnation method, the substrate powder of
substrate is soaked in the catalyst solution for certain time,
typically 30 minutes-1 hour (longer soaking time is required, if
the substrate is a porous).Then, the substrate is dried at
70.degree. C.-80.degree. C. for substantial time to ensure that the
solvent is completely evaporated. Increasing the drying temperature
may lead to agglomeration of catalyst particles which would effect
the growth of CNT adversely. Ion exchange method has typically been
used with powdered substrates for bulk synthesis. In this method,
the substrate powder is soaked in catalyst solution for 1-3 days
and the pH of the solution is adjusted to be in between 7-9 so that
it facilitates optimal deposition of catalyst ions on the
substrate.
[0086] Amount of catalyst and concentration of catalyst: The amount
of catalyst (i.e. supported catalyst powder or the size of
substrate impregnated with catalyst solution) governs the flow rate
of feed gas; greater amounts and bigger substrates needing higher
flow rates of precursor gas. Also, the concentration of catalyst
governs the growth of CNT. Generally, 2.5-10% by weight metal in
substrate is used for bulk synthesis. The concentration of catalyst
influences the density of CNT formed. However, if the concentration
of catalyst is too high, it leads to the formation of amorphous
carbon. This could be because of clusters of metal particles that
form at high temperatures, which are not capable of producing
CNT.
[0087] Size of catalyst particle: It is currently thought that the
size of catalyst particle governs the diameter of the nanotube, and
that the catalyst particle must fall within a narrow size range for
optimal activity. However, exemplifying the state of the art, a
consensus on whether the size of the particle controls the ID or OD
of the tube has not been made. Commonly, it is believed that the
catalyst particle size controls the inner diameter of the tube and
the outer diameter is controlled by the number of layers and
thickness of pyrolytic carbon, which is dependent on how long the
catalyst particle is active (as long as it is not completely
covered with carbon). From previous results obtained by various
authors, when catalyst salt solution is used, the average OD of the
CNT on a graphite of silica substrate is around 100 nm.
[0088] Pre-treatment of catalyst: After the catalyst is applied to
the substrate (either metallic form or a metal salt solution), it
is subjected to conditions that activate the catalyst particles.
Typically calcination in air or nitrogen at 500.degree. C.,
reduction in hydrogen at 600.degree. C. for varying durations is
used. Calcination decomposes the metal salt to its constituents so
that, additional groups (e.g. nitrate group in ferric nitrate) are
lost from the substrate. Depending on the calcination temperature
and atmosphere, the remaining metal ion stays in the ionic form or
reacts with oxygen to form some oxide. However, some researchers
have postulated that FeO (an oxide that can be formed by treating
iron with water vapor or wet nitrogen; it does not form directly on
heating iron in air) has greater catalytic activity than metallic
iron.
[0089] Growth Temperature: The growth temperature needed for the
growth of CNT is usually in the range of 700-800.degree. C. As
mentioned earlier, the growth of CNT is always accompanied by the
formation of amorphous carbon. There is a trade-off between the
quality of CNT and quantity of pure CNT. At lower temperatures,
around 650.degree. C., there is no amorphous carbon formed, but the
graphitization of the walls is not complete. The crystalline
graphite structure of the walls is not well-defined and these CNT
have poor quality. As the growth temperature is increased, the
walls are more turbostratic in nature and the structure of each
wall is well-defined; but, the amount of amorphous carbon also
increases. The temperature thus governs the diameter of the
nanotube, because it governs the thickness of pyrolytic carbon
deposited on their surface. The growth temperature does not
directly govern the length of the nanotubes, but controls the
activity of the catalyst particle. The catalyst particle remains
active as long as it is not enclosed by amorphous carbon, this in
turn depends on the growth temperature and composition of the
reactive gas mixture (i.e. concentration of the hydrocarbon gas in
the inert gas carrier).
[0090] Growth Pressure: Generally, in cases when a metal is sputter
coated on some substrate, the pressure in the reaction tube is
below atmospheric pressure (in the range of few torr) and methods
that use metal salt solutions are carried out at atmospheric
pressure. Not many studies have been conducted on the effect of
pressure on the growth of CNT. However, Ren et al. have reported
that the CNT yield increases as the pressure increases from 0.6
torr to 600 torr and then decreases as the pressure approaches
atmospheric pressure. However, graphitization of the walls is
better at 760 torr. As the pressure increases, the CNT get thicker
by increasing the number of graphene layers.
[0091] Carbon Precursor Gas: The amount of hydrocarbon needed in
the reaction depends on the amount of substrate/catalyst powder
present or the size of the substrate. For a bigger substrate, a
higher flow rate of gas is required. For a given amount of
catalyst, as the flow rate of hydrocarbon gas increases, the amount
of amorphous carbon formed also increases. There are, as yet, no
equations that govern the relation between the amount of catalyst
and required flow rate for optimal growth of CNT; each process will
require its own conditions and in traditional methods of VGCNT
growth fabrication, the variables cannot be "tuned" as is provided
by the present invention. Acetylene has, so far, been found to have
the highest activity amongst hydrocarbons in the formation of CNT.
However, other gases such as ethylene, propylene, benzene and
natural gas, have been found to be usable.
[0092] Duration of growth: The duration basically governs the
length of the CNT. It has been observed that there is formation of
CNT even during the first one minute of growth and as the duration
increases, the length of CNT continues to increase as long as the
catalyst particle is not deactivated (i.e., covered with amorphous
carbon). It was observed that a negligible amount of amorphous
carbon is formed during the first 30 minutes of growth. As the
duration increase beyond 1 hour, the amount of amorphous carbon
progressively increases. It has also been observed that the longest
tubes are often the thickest ones. Generally, reaction times
ranging from 30 minutes to 3 hours have been used depending on the
other growth parameters and experimental setup.
[0093] As a general rule, for the growth of CNT, the rates of
dissolution, diffusion and precipitation of carbon atoms in the
catalyst particles must match. At non-optimal conditions of
temperature, flow rates, the concentration of carbon atoms is
probably too high; the dissolving rate is higher than diffusion and
precipitation rates, resulting in an accumulation of the carbon
atoms on the top of the catalyst particles. The over-saturated
catalyst particles will lose their catalytic activity in a short
growth time and the yield of CNT is reduced.
[0094] Therefore, it should be appreciated that the ability to
provide preforms that have an optimal density or the desired size
and strength of CNT and that can be fabricated in a time efficient
manner is a matter of optimizing many variables. Further, it should
be appreciated that by "time efficient" the inventors mean, not
only in an economic sense but, more importantly in the sense that
each of the steps necessary takes place at an optimum time so as to
produce the desired nanotube type on the desired preform structure.
Rushing the process may lead to no CNT, reduced CNT or CNT with
excessive amorphous carbon residue, for example. Similarly,
delaying the process may, for example, result in a catalyst
solution that does not have the desired reactivity and/or CNT of
the inappropriate diameter.
[0095] The vapor grown carbon fibers are produced by contacting a
hydrocarbon gas with a catalytic particle under appropriate
reaction conditions and therefore this invention requires that the
continuous fiber preform be subjected to a process that yields
distributed catalytic particles within the preform. This can be
accomplished by numerous methods. For example, the catalyst may be
introduced into the preform by liquid or gas phase infusion of the
preform with a suitable catalyst particle, or by infusion with a
liquid or gas phase precursor solution that leads to the formation
of the catalyst particle in situ. The continuous fiber preform may
also be treated so that the catalyst is dispersed along and within
the preform prior to manufacturing of a more complex preform. For
example, a continuous yarn may be treated with catalyst solution
and subsequently woven or braided into a more complex preform. In
these manners, the metal catalyst particle or metal catalyst
particle yielding solution is distributed uniformly throughout the
preform.
[0096] Iron, nickel, cobalt, copper, chromium, or molybdenum
catalytic particles and mixtures thereof are useful for dispersing
on the continuous fiber preform to produce VGCNT when contacted
with an appropriate hydrocarbon gas under the appropriate
conditions. Iron, nickel, cobalt, copper, chromium, or molybdenum
compounds, and mixtures thereof are useful in the form of precursor
solutions for treating preforms. Non-limiting examples of such
solutions include the acetates, nitrates, and oxalates of iron,
nickel, and cobalt in solutions with water, alcohols, or mixtures
thereof. Organo-metallic compounds with iron, nickel, or cobalt
(such as ferrocene, nickelocene, and cobaltocene) and mixtures
thereof will also be useful as catalyst precursor solutions. In
general, the metal compound is dissolved in an appropriate solvent
at the desired concentration, and then the preform is dipped,
sprayed, or continuously passed through the solution followed by
heating to remove the solvent. These non-limiting illustrations are
methods of uniformly treating the preform with the catalyst
precursor solution. Modifications to this process obvious to those
skilled in the art are within the scope of this invention. For
example, treatment of the preform with a sulfur bearing compound
(e.g. thiophene) in addition to the catalyst may enhance the
catalytic activity. Also, other additives may be used in the
catalyst precursor solution to enhance wetting of the substrate by
the solution (i.e. surfactants, wetting agents, soluble polymers
such as PVAC or BTDE).
[0097] According to some exemplary embodiments of the invention,
iron (III) nitrate nonahydrate (ferric nitrate) solutions in
ethanol at a concentration ranging from 1 mM to 200 mM are
effective as catalyst precursor treatments for continuous fiber
preforms. More preferably, ethanol solutions of ferric nitrate with
concentrations in the range of 25 mM to 125 mM have been shown to
be very effective as a catalyst precursor treatment for carbon
fiber preforms.
[0098] After treatment with the desired catalytic particle or
catalyst precursor the preform is heated batchwise or in a
continuous mode, to decompose the metal compound and yield the
metal catalyst particle. This may be performed by heating at
temperatures from 100.degree. C. to 1000.degree. C., in some cases
in an oxidizing atmosphere. Preferably, this is performed in air at
temperatures from 300.degree. C. to 800.degree. C. to yield an
oxidized metal catalytic particle. Oxidation pre-treatment has been
discovered to give much higher VGCNT yield on continuous carbon
fiber preforms.
[0099] In various other embodiments, after treatment with the
desired catalytic particle or catalyst precursor, the preform is
subsequently treated in a flowing gas mixture to reduce the
catalyst to a metallic particle. Preferably, this is done in a
hydrogen/nitrogen or hydrogen/argon gas mixture using hydrogen from
1% to 100% of the gas mixture at a temperature from 100.degree. C.
to 1200.degree. C. for a period of time from 1 minute to 100 hours.
Most preferably the hydrogen is at 10% of the gas mixture, the
temperature is in the range from 400.degree. C. to 800.degree. C.
and the time is in the range of 1 hour to 12 hours.
[0100] The vapor grown carbon fibers are then produced on the
continuous fiber preforms from the distributed catalyst particles
by contacting a gas phase hydrocarbon or hydrocarbon gas mixture
with the preform at a temperature from 500.degree. C. to
1200.degree. C. The vapor grown carbon fibers grow from the
catalyst particles within the woven or braided composite preform
resulting in a tangled mass of vapor grown carbon fibers
infiltrated in the continuous fiber preform. The vapor grown carbon
fibers fill void spaces between the continuous fibers in the
preform and may exhibit partial orientation and alignment depending
on the geometry and architecture of the preform. If the composite
preform is constructed from graphite or carbon fibers the vapor
grown carbon fibers may fuse to the fibers leading to further
enhancement of properties. In particular embodiments, the
hydrocarbon gas is modulated or pulsed during the VGCNT growth
process by turning the hydrocarbon gas flow on and off at periodic
intervals while maintaining the flow of inert gas. The inventors
have made the unexpected finding that, in some embodiments, this
"pulsed" approach increases yield of VGCNT on carbon fiber
preforms.
[0101] In some exemplary embodiments, the inventors have found that
for some continuous preforms, it can be advantageous to initiate
heating of the preform in an air atmosphere to oxidize the metal
catalytic particle and carbon fiber surface to increase yield and
improve the resulting morphology of the VGCNT infused preform.
Contrary to literature reports of VGCNT growth on graphite
substrates where the universal procedure is to purge the substrate
with an inert gas prior to heating, the inventors have discovered
the unexpected result that much higher yield of VGCNT and higher
aspect ratio is obtained if air is maintained initially in the
reactor vessel in a manner to allow partial oxidation of the
catalytic particle and substrate surface. The enhanced growth may
also be a result of burning off excess residual carbon from
decomposition of the catalyst precursor.
[0102] The hydrocarbon gas can include acetylene, methane, propane,
ethane, ethylene, benzene, natural gas or mixtures thereof In some
exemplary embodiments, the hydrocarbon gas is acetylene and
nitrogen or argon gas is mixed with the acetylene prior to
introduction in the growth furnace containing the composite
preform, the growth temperature is between 700.degree. C. and
850.degree. C., the reaction time is between 15 minutes and 2 hours
and the pressure is atmospheric. More preferably, the gas mixture
is in the range of 1% to 20% acetylene and 99% to 80% nitrogen or
argon, the temperature is between 750.degree. C. and 850.degree. C.
and the reaction time is 30 to 120 minutes. Most preferably, the
gas mixture is in the range of 1% to 10% acetylene and 99% to 90%
nitrogen, the temperature is between 750.degree. C. and 850.degree.
C. and the reaction time is 30 to 60 minutes.
[0103] The VGCNT infused continuous fiber preforms may subsequently
be subjected to processing operations known to artisans such as
heat treatment, solvent wash, and other treatments designed to
remove the metal catalyst from the preform and change the chemical
composition and physical characteristics of the vapor grown carbon
fiber surface. Such surface treatment may be desirable to increase
adhesion to a polymer or other matrix material in composite
materials manufactured from these preforms, or to make the VGCNT
infused preform more suitable for an application as an electrode,
filter media, remediation media, gas storage media, or support for
catalysis or cell growth and tissue regeneration.
[0104] As previously mentioned, after the VGCNT are grown in the
preform it may be desirable to treat the preform with an aqueous
solution of an inorganic acid, such as a mineral acid, to remove
excess catalyst particles, if present, and to improve the bonding
characteristics of the VGCNT infused preform. Non-limiting examples
of suitable mineral acids include sulfuric acid, nitric acid, and
hydrochloric acid. Preferred is nitric or sulfuric acid, or a
sulfuric acid treatment followed by a nitric acid treatment.
[0105] In some exemplary embodiments, embodiment the continuous
fiber preform is produced from either continuous polyacrylonitrile
(PAN) or pitch carbon fibers (e.g. commercially sold as IM7.RTM.,
AS4, T300, T700, PANEX.RTM. 33 (McKechnie, UK LTD, West Midlands,
UK), T40-800, T650-35, YS-90A, CARBOFLEX.RTM. (Imerys Minerals LTD,
Cornwall, UK) and vapor grown carbon fibers produced in situ in the
continuous carbon fiber preform.
[0106] In various exemplary embodiments, the woven or braided
preform is produced from commercially available, non-carbon fiber
continuous fiber such as E-glass, S-glass, quartz, metal or ceramic
and vapor grown carbon fibers are produced in situ in the
continuous fiber preform. In these embodiments, the vapor grown
carbon fibers can impart desirable mechanical, electrical and
thermal characteristics to composites manufactured from these
preforms. These substrate fibers in general have poor thermal and
electrical conductivity. For example, infusion of the continuous
fiber preform with VGCNT as described in this invention imparts
electrical conductivity at extremely low levels of VGCNT.
[0107] Polymer matrix composite articles can be manufactured from
these vapor grown carbon fiber reinforced composite preforms by
infusing the preform with suitable matrix materials. In a preferred
embodiment, the resulting vapor grown carbon fiber reinforced
preforms are subsequently infused by a thermoplastic polymer in the
molten state, a suitable low viscosity thermoset polymer resin, a
polymer resin solution, powdered polymer particle dispersion, or
any other means know by artisans to infuse a polymer into a
continuous reinforcing fiber preform. Such polymer resins,
thermoplastics and the like are commercially available from, for
example, Solvay Advanced Polymers, L.L.C, Belgium, However, it
should be appreciated that the manufacture of composites from the
vapor grown carbon fiber reinforced preforms according to the
present invention is not limited to polymer matrices. For example,
VGCNT composites can exhibit useful properties when infused by
metals, ceramics and ceramic precursors, pitches and other carbon
precursors.
[0108] In various exemplary embodiments, the woven, braided or
other preform is produced from either continuous PAN or pitch
carbon fibers (e.g. commercially sold as IM7, AS4, T300, T700,
PANEX.RTM. 33 (McKechnie, UK LTD, West Midlands, UK), T40-800,
T650-35, YS-90A, CARBOFLEX.RTM. (Imerys Minerals LTD, Cornwall,
UK), and other equivalent materials) and vapor grown carbon fibers
produced in situ in the continuous carbon fiber preform, and the
resulting vapor grown carbon fiber reinforced continuous fiber
preform is infused with a suitable thermoset polymer resin and
thermally processed into a finished composite article with useful
properties. Examples of such commercially available thermoset
polymer resins include P.sup.2SI.TM. 635LM, P.sup.2SI.TM. T3, OR
P.sup.2SI.TM. 700LM (Performance Polymer Solutions, Inc.,
Ohio).
[0109] Those of skill in the art will appreciate that according to
the process and method described herein, the continuous fiber
preform is in transit continuously during the growth process and
being in complete and intimate contact with the environment of the
growth furnace. Without being held to any particular theory, the
continuous process of the present invention may therefore allow
more complete and dense growth of VGCNT because every catalytic
particle is intimately exposed to the growth environment and allows
access of the gas mixtures to the interior of the fiber.
[0110] The following paragraphs enumerated consecutively from 1
through 50 provide for various aspects of the present invention. In
one embodiment, in a first paragraph (1), the present invention
provides: A continuous process for producing a carbon nanotube
reinforced continuous fiber preform useful in the manufacture of
carbon nanotube reinforced composite articles, comprising steps of:
(a) dispersing a catalyst precursor throughout a continuous fiber
preform; (b) converting the catalyst precursor into catalytic
particles, the catalytic particles dispersed throughout the
continuous fiber preform; (c) contacting the continuous fiber
preform containing the catalytic particles with a hydrocarbon
precursor gas; and (d) continually moving the treated preform
through a growth furnace; whereby vapor grown carbon nanotubes are
formed in situ at the catalytic particles dispersed throughout the
continuous fiber preform to yield a carbon nanotube reinforced
continuous fiber preform.
[0111] 2. The process of paragraph 1, wherein the catalyst
precursor comprises a solution of iron, nickel, cobalt, copper,
chromium, molybdenum, a salt or a mixture thereof
[0112] 3. The process of paragraph 2, wherein the solvent for the
catalyst precursor is, an alcohol, acetone, ethanol, isopropanol,
hexane, methanol, water or mixtures thereof.
[0113] 4. The process of paragraphs 1-3, wherein the catalyst
precursor is iron acetate, iron nitrate, iron oxalate, nickel
acetate, nickel nitrate, nickel oxalate, cobalt acetate, cobalt
nitrate, cobalt oxalate, or a mixture thereof.
[0114] 5. The process of paragraph 1, wherein the catalyst
precursor is a solution of iron (III) nitrate nonahydrate (ferric
nitrate) in ethanol, acetone or ethanol/acetone mixture.
[0115] 6. The process of paragraphs 1-5, wherein the continuous
fiber preform comprises a carbon preform, ceramic preform, glass
preform, quartz preform, a graphite preform, a metal preform or
combinations thereof.
[0116] 7. The process of paragraphs 1-6, wherein the preform is a
continuous multi-filament, braid, weave, yarn or tow.
[0117] 8. The process of paragraphs 1-7, wherein the catalyst
precursor treated preform is pyrolyzed to form catalytic particles
within the preform in a pyrolysis furnace.
[0118] 9. The process of paragraphs 1-8, wherein the pyrolysis
further removes organic content from the preform.
[0119] 10. The process of paragraphs 1-8, wherein the pyrolysis
furnace includes and inlet and an outlet and a mechanism for
continuously taking up the continuous preform as it exits the
furnace.
[0120] 11. The process of paragraphs 8-10, wherein the pyrolysis
furnace further includes a mechanism for continuously belaying the
continuous fiber preform into the pyrolysis furnace and wherein the
rate of belaying and taking up are approximately equal.
[0121] 12. The process of paragraphs 8-11, wherein the pyrolysis of
the catalyst precursor takes place at between about 300.degree. C.
and 900.degree. C.
[0122] 13. The process of paragraphs 8-12, wherein the pyrolysis of
the catalyst precursor takes place in an inert or oxidizing gas
atmosphere.
[0123] 14. The process of paragraph 13, wherein the pyrolysis of
the catalyst precursor takes place in an argon or nitrogen
atmosphere.
[0124] 15. The process of paragraphs 8-14, wherein the pyrolysis of
the catalyst precursor takes place from, between 1 second to 30
minutes.
[0125] 16. The process of paragraphs 8-15, wherein the pyrolysis of
the catalyst precursor takes place at from about 1 minute to about
15 minutes.
[0126] 17. The process of paragraphs 8-16, wherein the pyrolysis of
the catalyst precursor takes place at from about 500.degree. C. to
about 600.degree. C.
[0127] 18. The process of paragraphs 8-17, wherein the pyrolyzed
preform is fed in to a front-end of a growth furnace with a
precursor gas to induce growth of carbon nanotubes.
[0128] 19. The process of paragraphs 18, wherein the fed-in preform
is taken-up at a rear-end of the furnace.
[0129] 20. The process of paragraphs 18-19, wherein the residence
time of the preform through the growth furnace is approximately
between about 1 minute to 1000 minutes.
[0130] 21. The process of paragraphs 20, wherein the residence time
of the preform through the growth furnace is between about 1
minutes and 120 minutes.
[0131] 22. The process of paragraphs 18-21, wherein the pyrolyzed
preform has a heat treatment step prior to induction of nanotube
growth.
[0132] 23. The process of paragraphs 18-22, wherein the heat
treatment step and the nanotube growth step occur in the same
furnace.
[0133] 24. The process of paragraphs 18-23, wherein the nanotube
growth step occurs sequentially after the heat treatment step.
[0134] 25. The process of paragraphs 18-24, wherein the heat
treatment step occurs at a temperature of about approximately
600.degree. C. to about 900.degree. C.
[0135] 26. The process of paragraphs 18-25, wherein the heat
treatment step occurs at a temperature of about 800.degree. C.
[0136] 27. The process of paragraphs 22-26, wherein the heat
treatment step happens in an inert atmosphere.
[0137] 28. The process of paragraphs 18-27, wherein the nanotube
growth step occurs at a temperature of about approximately
700.degree. C. to about 950.degree. C.
[0138] 29. The process of paragraphs 18-28, wherein the nanotube
growth step occurs at a temperature of about 750.degree. C. to
about 850.degree. C.
[0139] 30. The process of paragraphs 18-29, wherein the precursor
gas has a flow velocity in the furnace of approximately about 10 to
1000 cm/min.
[0140] 31. The process of paragraphs 18-30, wherein the flow
velocity in the furnace is approximately about 100 to 150
cm/min.
[0141] 32. The process of paragraphs 18-31, wherein the precursor
gas is provided in a reactive gas composition comprising about
approximately 0.1% to 10% hydrocarbon precursor gas in 99.9% to 90%
inert gas.
[0142] 33. The process of paragraphs 18-32, wherein the precursor
gas is provided in a reactive gas composition comprising about
approximately 0.5% to 2% hydrocarbon precursor gas in 99.5% to 98%
inert gas.
[0143] 34. The process of paragraphs 18-33, wherein the precursor
gas is provided in a reactive gas composition comprising about
approximately 1% hydrocarbon precursor gas in 99% inert gas.
[0144] 35. The process of paragraphs 18-34, wherein the reactive
gas composition is 1% acetylene in nitrogen.
[0145] 36. The process of paragraphs 22-35, wherein the wherein the
growth furnace is a two-zone furnace and heat treatment occurs in a
first zone and nanotube growth occurs in a second zone.
[0146] 37. The process of paragraphs 22-36, wherein each zone has a
different temperature.
[0147] 38. The process of paragraphs 22-37, wherein the hydrocarbon
precursor gas is entered into the furnace after the heat treatment
zone.
[0148] 39. The process of paragraphs 22-36, wherein the hydrocarbon
precursor is entered into the furnace before the heat treatment
zone but is not mixed with the purge gas until the second zone.
[0149] 40. The process of paragraphs 1-17 wherein step (b) is
carried out under reducing conditions.
[0150] 41. The process of paragraphs 1-40, wherein the hydrocarbon
precursor gas is, acetylene, methane, propane, ethane, ethylene,
benzene, natural gas or mixtures thereof.
[0151] 42. The process of claims 1, wherein multiple preforms are
processed concurrently.
[0152] 42. A carbon nanotube reinforced continuous fiber preform
produced by the process of paragraphs 1-41.
[0153] 43. The carbon nanotube reinforced continuous fiber preform
of paragraph 42, wherein the fiber preform is carbon, quartz,
glass, ceramic or metal multi filament yarn, tow, braid or
weave.
[0154] 44. A furnace useful for fabricating a continuous preform
having vapor grown carbon nanotubes grown thereon in a continuous
process comprising: a tube furnace having an inlet and an outlet
and a growth zone; a mechanism for continuously feeding the preform
into the inlet and a mechanism for continuously taking up the
preform at the outlet; and wherein the rate of feeding-in and
taking-up are approximately equal such that the continuous fiber
preform is continuously fed into the furnace for the continuous
process of growing carbon nanotubes, in situ on the continuous
preform.
[0155] 45. The furnace of paragraph 44, wherein an inert gas purge
is applied to the furnace at the inlet.
[0156] 46. The furnace of paragraphs 44-45, wherein the furnace
further includes heat treatment zone.
[0157] 47 The furnace of paragraphs 44-46, wherein a hydrocarbon
precursor gas is entered into the furnace after the first zone and
before the second zone.
[0158] 48. The furnace of paragraphs 44-46, wherein the hydrocarbon
precursor gas is entered into the furnace before the first zone and
mixed with the purge gas before the growth zone.
[0159] 49. A process for providing a carbon nanotube reinforced
composite article comprising steps of: (a) dispersing a catalyst
precursor throughout a continuous fiber preform; (b) converting the
catalyst precursor into catalytic particles, the catalytic
particles dispersed throughout the continuous fiber preform; (c)
continually moving the treated preform through a pyrolysis furnace;
(d) contacting the continuous fiber preform containing the
catalytic particles with a hydrocarbon precursor gas to yield a
carbon reinforced continuous fiber preform; wherein vapor grown
carbon fibers are deposited in situ at the catalytic particles
throughout the continuous fiber preform to yield a carbon
reinforced continuous fiber preform; and (e) infusing the carbon
reinforced continuous fiber preform with a thermoplastic or
thermoset polymer, thermoplastic or thermoset polymer resin, metal,
ceramic, ceramic precursor, or amorphous glass to provide a carbon
nanotube reinforced composite article.
[0160] A carbon nanotube reinforced composite article produced by a
process according to paragraph 49.
EXAMPLES
[0161] This invention is illustrated in the examples which follow.
The examples are set forth to aid in an understanding of the
invention but are not intended to, and should not be construed to
limit in any way the invention as set forth in the claims which
follow thereafter.
Example 1
Carbon Fiber Textile Infusion with VGCNT
[0162] A piece of plain weave polyacrylonitrile (PAN) carbon fiber
cloth was desized by solvent wash with toluene and acetone followed
by oven drying. The sample was then immersed in a 125 mM solution
of ferric nitrate in ethanol, and dried at 80.degree. C. and placed
in a 50 mm diameter tube furnace. The tube furnace was immediately
heated to 800.degree. C. and nitrogen flow of 90 sccm was started
when the tube furnace temperature reached 100.degree. C. After 15
minutes at 800.degree. C. 5 sccm of acetylene was started and the
nitrogen flow was reduced to 75 sccm. After 60 minutes the
acetylene was turned off and the oven was cooled to 200.degree. C.
under nitrogen flow of 75 sccm. SEM images of the resulting VGCNT
infused preform are shown in FIGS. 1A and 1B. In FIG. 1A, the
continuous carbon fibers of the preform are clearly visible with
the mass of entangled VGCNT infused into the preform. In FIG. 1B at
higher magnification, the morphology of the well-formed VGCNT are
shown fused to the continuous carbon fiber surface.
Example 2
Carbon Fiber Textile Infusion with VGCNT
[0163] A piece of plain weave PAN carbon fiber cloth was desized by
solvent wash with toluene and acetone followed by oven drying. The
sample was then immersed in a 100 mM solution of ferric nitrate in
ethanol, and dried at 80.degree. C. The sample was then heated at
300.degree. C. for 30 hours in an air convection oven, cooled, and
placed in a 50 mm diameter tube furnace. The tube furnace was
heated to 750.degree. C. and when it reached 600.degree. C.
nitrogen flow of 90 sccm was started. After 15 minutes at
750.degree. C. 5 sccm of acetylene was started and the nitrogen
flow was increased to 250 sccm. After 60 minutes the acetylene was
turned off and the oven was cooled to 200.degree. C. under nitrogen
flow of 90 sccm. An SEM image of the resulting VGCNT infused
preform is shown in FIG. 2. In FIG. 2, the continuous carbon fibers
of the preform are clearly visible with the mass of entangled VGCNT
infused into the preform.
Example 3
Carbon Fiber Textile Infusion with VGCNT
[0164] A piece of plain weave PAN carbon fiber cloth was desized by
solvent wash with toluene and acetone followed by oven drying. The
sample was then immersed in a freshly prepared 100 mM solution of
ferric nitrate in ethanol, and dried at 80.degree. C. The sample
was then heated at 300.degree. C. for 30 hours in an air convection
oven, cooled, and placed in a 50 mm tube furnace. The tube furnace
was heated to 750.degree. C. and when it reached 600.degree. C.
nitrogen flow of 90 sccm was started. After 15 minutes at
750.degree. C. 5 sccm of acetylene was started and the nitrogen
flow was increased to 250 sccm. After 60 minutes the acetylene was
turned off and the oven was cooled to 200.degree. C. under nitrogen
flow of 90 sccm. An SEM image of the resulting VGCNT infused
preform is shown in FIG. 3. In FIG. 3, the continuous carbon fibers
of the preform are obscured by the high yield dense growth mass of
entangled VGCNT infused into the preform.
Example 4
Carbon Fiber Textile Infusion with VGCNT
[0165] A piece of plain weave PAN carbon fiber cloth was desized by
solvent wash with toluene and acetone followed by oven drying. The
sample was then immersed in a freshly prepared 25 mM solution of
ferric nitrate in ethanol, and dried at 80.degree. C. then placed
in a 50 mm tube furnace. The tube furnace was heated to 750.degree.
C. and when it reached 100.degree. C. nitrogen flow of 90 sccm was
started. After 15 minutes at 750.degree. C. 5 sccm of acetylene was
started and the nitrogen flow was reduced to 75 sccm. After 30
minutes the acetylene was turned off and the oven was cooled to
200.degree. C. under nitrogen flow of 90 sccm. An SEM image of the
resulting VGCNT infused preform is shown in FIG. 4. In FIG. 4, the
continuous carbon fibers of the preform are visible along with the
high yield dense growth mass of entangled VGCNT infused into the
preform.
Example 5
Carbon Fiber Textile Infusion with VGCNT Using a Reduced
Catalyst
[0166] A piece of plain weave PAN carbon fiber cloth was desized by
solvent wash with toluene and acetone followed by oven drying. The
sample was then immersed in a freshly prepared 75 mM solution of
ferric nitrate in ethanol, and dried at 80.degree. C. The sample
was subsequently was heated in air at 300.degree. C. to decompose
and oxidize the iron catalyst. The sample was then placed in a 50
mm tube furnace and heated to 500.degree. C. under a nitrogen flow
of 250 sccm. At 500.degree. C. the nitrogen flow was reduced to 150
sccm and 15 sccm of hydrogen was introduced. These conditions were
held for four hours to reduce the catalytic particles to metallic
iron. The hydrogen flow was then stopped and the nitrogen increased
to 250 sccm and the tube furnace cooled to 50.degree. C. Under a
flow of 250 sccm of nitrogen the tube furnace was heated to
700.degree. C. When it reached 700.degree. C. the nitrogen flow was
reduced to 90 sccm. After 15 minutes at 700.degree. C. 5 sccm of
acetylene was started and the nitrogen flow was reduced to 75 sccm.
After 60 minutes the acetylene was turned off and the oven was
cooled to 200.degree. C. under nitrogen flow of 90 sccm. An SEM
image of the resulting VGCNT infused preform is shown in FIG. 5. In
FIG. 5, the continuous carbon fibers of the preform are visible
along with the dense growth of low aspect ratio VGCNT infused into
the preform.
Example 6
Conductive Composite Fabricated from Quartz Fabric Infused with
VGCNT
[0167] A piece of high temperature resistant fabric woven with
quartz glass yarns (available from JPS Composite Materials
Corporation under federally-registered trademark ASTROQUARTZ
II.RTM., style 525) was solution coated with a 75 mM ferric nitrate
solution in ethanol and dried for 1 hour at 80.degree. C. The
sample was then weighed, placed in a 50 mm tube furnace, nitrogen
flow was started at 75 sccm, and heated to 750.degree. C. After 15
minutes at 750.degree. C. acetylene flow of 15 sccm was started to
initiate VGCNT growth. After 1 hour the acetylene flow was stopped
and the tube furnace was cooled to room temperature. The resulting
fabric sample had a uniform metallic, shining dark gray appearance
and was found to have increased in mass by 0.15%. The sample was
then cut into strips 12 cm wide and 60 cm long aligned in the warp
direction and a composite fabricated by coating 6 of these strips
with an epoxy resin (EPON.RTM. 862/Cure Agent W) and compression
molding in a heated press for 4 hours at 250.degree. F. and 2 hours
at 350.degree. F. The sample was then removed from the mold and the
resistance measured at several points in both the warp and fill
directions. The average resistivity between probes 1 cm apart in
the warp direction was found to be 0.130 .OMEGA.meter and the
average resistivity between probes 1 cm apart in the weft (fill)
direction was found to be 0.135 .OMEGA.meter. This data indicates
that the conductivity of the composite material was uniform in the
plane of the fabric and was increased by approximately a factor of
10.sup.10 to 10.sup.14 at a composite mass fraction of about 0.1%
VGCNT.
Example 7
Continuous Process for the Fabrication of a Continuous Preform
[0168] Any suitable continuous media such as: multi-filament yarns;
mono-filaments; continuous woven or braided constructions prepared
from yarns. Materials for the preforms include ceramic fibers,
graphite and carbon fibers, glass and quartz fibers, and metals.
The inventors have successfully demonstrated the process on several
types of carbon and graphite fibers, quartz fibers, and several
grades of SiC yarns including those sold under the tradenames
Nicalon.TM.-CG, Hi-Nicalon.TM., and Hi-Nicalon.TM. (Nippon Carbon
Co.) coated with proprietary coatings for ceramic matrix composite
(CMC) processing. See, Table 1.
TABLE-US-00002 TABLE 1 Continuous Fiber Preform Manufacturer
Nicalon .TM.-CG (SiC) Nippon Chemical Company LTD, Japan Hi-Nicalon
.TM. (SiC) Nippon Chemical Company LTD, Japan Hi-Nicalon .TM.
(SiC)With Coatings Nippon Chemical Company LTD, Japan Astroquartz
II, Style 525 Textile (Quartz) JPS Composite Materials Corp. AS4
Carbon Fiber (Carbon) Hexcel Corp. AU4 Carbon Fiber (Carbon) Hexcel
Corp. T650-35 Carbon Fiber (Carbon) Cytec Carbon Fibers LLC IM7
Carbon Fiber (Carbon) Hexcel Corp.
[0169] The continuous process for the production of nanotube
infused preforms consists of four distinct steps: 1) treatment of
the preform with catalyst particle precursor solution; 2)
"burn-off" of any organic preform sizing and decomposition of the
catalyst precursor solution and to form active catalytic particles;
3) heat treatment of the catalyzed preform; and 4) contacting the
catalyst treated preform with a decomposed hydrocarbon precursor
gas to induce catalytic carbon nanotube growth.
[0170] Step 1: Yarn Catalyst Treatment
[0171] FIG. 6 is a schematic of one exemplary embodiment of the
invention showing a method for the continuous treatment of the
preform with catalyst solution. Several types of catalyst bearing
salt solutions have been evaluated including Iron, Nickel, and
Cobalt compounds, See, for example, Table 2, following paragraph
[0213] below. In one exemplary embodiment the solution is a
solution prepared from Iron(III)Nitrate,
(Fe(NO.sub.3).sub.3.9H.sub.2O. Several concentrations (10 mM to 250
mM) have been evaluated and 100 mM has been found to yield optimal
growth. The inventors have investigated several different solvents
(water, acetone, ethanol, isopropanol, hexanes, methanol, and
mixtures thereof) for the catalyst salt. While each has shown the
ability to infuse the preform with catalyst precursors, in some
exemplary embodiments, solvent systems which wick rapidly into the
fibrous preforms and dry rapidly in air at room temperature have so
far been shown to be most optimal. Such systems include, for
example dipping/drying process using a 50:50 (volume:volume)
mixture of acetone and ethanol for the catalyst precursor
solution.
[0172] Step 2: Catalyst Pre-Heat-Treatment Pyrolysis Step
[0173] Continuous fiber preforms are usually sized at a level of
0.5 wt % to 3 wt % to facilitate weaving and handling
characteristics. Generally, the inventors have found that it is
desirable to use the preforms as received for best handling
characteristics and economy. This organic sizing content may not
interfere with the catalyst solution treatments, but was found to
inhibit nanotube growth in subsequent steps unless it is pyrolyzed.
Compared to a "batch type" process, this aspect of the processing
is unique. In a continuous process, the "up stream" operations
affect the "down stream" operations. For batch-wise processing the
pyrolysis step can be eliminated since the off-gassing due to
pyrolysis of the organic content is carried away from the preform
by the inert gas flow. Additionally, without being held to any
particular theory, the inventors postulate that the iron (III)
nitrate decomposes during this step to yield an iron oxide
catalytic particle (the exact nature of which has not been
determined). The inventors intentionally allow the catalyst to
oxidize since the yarns can be stored and handled in the
atmosphere. It should be appreciated however, that the continuous
process is not hindered by keeping the catalyst in the unoxidized
state. Many other nanotube techniques utilize a reduction step
where any oxides of the metal particles are reduced to elemental
metal particles. While the inventors have not found such an
explicit reduction step necessary, it is within the scope of the
invention. Without being held to any particular theory, it is
likely that carbon deposits from pyrolysis, as well as carbon
present on the continuous fiber preform surface, reduce oxides of
the iron upon heating to yield elemental metal catalytic particles.
FIG. 7 is a schematic illustrating one exemplary embodiment of the
current invention showing a method for the continuous pyrolysis of
the catalyst solution treated preform. In various exemplary
embodiments, heat treatment is carried out in a nitrogen atmosphere
between 300.degree. C. and 900.degree. C., for 1 minute to 15
minutes residence. In some exemplary embodiments heat treatment
conditions are between 500.degree. C. and 600.degree. C. in the
range of 1 minute to 15 minutes. The pyrolyzed preforms are stable
in ambient conditions and atmospheric exposure does not adversely
affect subsequent processing.
[0174] Step 3: Heat Treatment of Catalyzed Preform
[0175] In various exemplary embodiments, the heat treatment step
and nanotube growth are carried out sequentially in a closed system
as shown in FIGS. 3 and 4. The inventors have found that the heat
treatment step yields better growth of nanotubes on the continuous
preforms. While the exact reason for improvement has not been
determined, the inventors speculate that the iron may reduce at
these temperatures, or that reactions with carbon present from
pyrolysis reacts with the oxide particles to yield an active
catalytic particle. The heat treatment step has been carried out
over a range of temperatures from 600.degree. C. to 900.degree. C.
and 800.degree. C. determined to be the preferred temperature. The
heat treatment is carried out in an inert gas atmosphere (any inert
gas can be used however, nitrogen is generally preferred due to its
less expensive cost).
[0176] In various exemplary embodiments according to the invention,
the preform is not allowed to cool down after the heat treatment
step. A schematic of this embodiment is shown in FIG. 9. Generally,
the inventors have found that the result of this modification is
consistently longer and denser nanotube growth on the preform fiber
surfaces. However, those of skill in the art will recognize that,
according to this embodiment, the hydrocarbon precursor gas should
be introduced prior to the heat treatment zone and this will
generally require a mixing system (shown as baffles) to be
incorporated to achieve a well-mixed homogeneous reactive gas
composition in the growth furnace.
[0177] Step 4: Nanotube Growth
[0178] The nanotube growth on the preform surfaces is accomplished
by introducing a hydrocarbon feed or precursor gas after the heat
treatment step. In various exemplary embodiments, acetylene
(C.sub.2H.sub.2) is the preferred feed gas however; the inventors
have tried a variety of precursor gases and found that they are
capable of forming VGCNT including acetylene, methane, propane,
ethane, ethylene, benzene, natural gas. The preferred reactive gas
composition for nanotube growth is 1% C.sub.2H.sub.2 in nitrogen
(nominally 5 sccm C.sub.2H.sub.2, 500 sccm N.sub.2 in a one-inch
tube furnace). The gas composition is critical for optimal nanotube
growth and the gas flow rate is also a critical aspect of the
growth process. The preferred bulk gas velocity into the reactor is
approximately 100 cm/min and in the tube furnace the velocity is
approximately 300 cm/min (adjusted for gas expansion at reaction
temperatures). In some exemplary embodiments, the preferred
nanotube growth temperature is between 700.degree. C. and
950.degree. C. with the most preferred temperature approximately
850.degree. C.
Example 8
Continuous Growth of Carbon Nanotubes on a 60 cm Length of Silicon
Carbide Fiber Yarn (Hi-Nicalon.TM.) at Atmospheric Pressure
[0179] A 100 mM Ferric Nitrate Nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) solution in an acetone and ethanol
mixture (50/50) was prepared and allowed to stand for 24 hours. A
60 cm length of as-received silicon carbide fiber yarn
(Hi-Nicalon.TM.) was used for the continuous CNT growth process.
The silicon carbide fiber yarn (Hi-Nicalon.TM.) was soaked in the
catalyst solution for 5 minutes and hung vertically to dry at room
temperature. The dry catalyst treated yarn was then bonded to a 305
cm length of untreated ceramic grade silicon carbide fiber yarn
(Nicalon.TM. CG) that served as a leader to continuously pull the
catalyst treated yarn through the growth furnace. The treated
silicon carbide fiber yarn (Hi-Nicalon.TM.) was then placed into a
25 mm diameter quartz tube in a tube furnace at 500.degree. C.
under a nitrogen flow of 500 sccm for pyrolysis of the catalyst at
a residence time of 15 minutes and the "downstream" end of the
quartz tube was open to the atmosphere. The carbon nanotube growth
on the continuous yarn was conducted in a 25 mm diameter quartz
tube reactor that was mounted in a two-zone tube furnace. The
furnace was 80 cm in overall length with each zone 40 cm in length.
The spool of yarn was placed into a container fitted with a
nitrogen gas inlet that was sealed to the quartz reaction tube at
the upstream end of the tube such that the untreated ceramic grade
silicon carbide fiber yarn (Nicalon.TM. CG) leader was fed down the
tube reactor through the furnaces to be used to pull the continuous
treated silicon carbide fiber yarn (Hi-Nicalon.TM.) through. In
this way the inert nitrogen gas purges the yarn chamber and then
flows down the reaction tube through the furnace and out the
downstream end open to the atmosphere. The first zone of the
furnace was set to a temperature of 800.degree. C. and the second
set to a temperature of 820.degree. C. Nitrogen flow was set to 500
sccm in the reactor system and maintained throughout the entire
process. After a period of 15 minutes to allow the reactor system
to completely purge, the yarn was pulled through the entire length
of the furnaces at a speed of 1.27 cm/minute. When the free end of
the silicon carbide fiber yarn had entered the tube furnace such
that the 60 cm length of yarn was in the tube furnace, high purity
acetylene was introduced upstream of the tube furnace at a flow
rate of 5.0 sccm (1% concentration of acetylene in nitrogen). This
process created a gradient of heat treatment and growth exposure
times along the length of the catalyst treated silicon carbide
fiber yarn. When the silicon carbide fiber yarn exited the tube
furnace, the acetylene flow was stopped and the reactor cooled
under nitrogen purge.
[0180] Carbon nanotubes were found grown from the surfaces of the
individual filaments within the silicon carbide fiber yarn and
along the length of the yarn. The surface density and the
characteristics of the carbon nanotubes varied depending on the
length of time each segment spent respectively in the zones of the
furnaces and the length of growth time with acetylene present. In
FIGS. 10A and 10B, the scanning electron microscope images show the
nanotube growth for the segment of silicon carbide fiber yarn
preform 46 cm from the leading edge (FIG. 10A) and 5 cm from the
leading edge (FIG. 10B).
Example 9
Continuous Growth of Carbon Nanotubes on AU4-12K Carbon Fiber Yarn
at Atmospheric Pressure
[0181] A 100 mM Ferric Nitrate Nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) solution in an acetone and ethanol
mixture (50/50) was prepared and allowed to stand for 24 hours. A
60 cm length of as-received AU4-12K un-sized yarn was used.
(Unsized AU4 12K (12000 filaments per tow) carbon fiber available
from Hexcel Corp. Stamford, Conn.) The AU4-12K un-sized yarn was
soaked in the catalyst solution for 5 minutes and hung vertically
to dry at room temperature. The dry catalyst treated yarn was then
bonded to a 305 cm length of untreated T650-35 6K yarn (Cytec
Carbon Fibers LLC, Piedmont, S.C.) that served as a leader to
continuously pull yarn through the reactor (in the same manner as
described in Example 1). The treated AU4-12K yarn was then pulled
continuously through a 75 cm tube furnace at 5 cm/minute in a 25 mm
diameter quartz tube at 500.degree. C. under a nitrogen flow of 500
sccm for pyrolysis of the catalyst for a residence time of 15
minutes. The "downstream" end of the quartz tube was left open to
the atmosphere. The carbon nanotube growth on the continuous
AU4-12K yarn was conducted in a 25 mm diameter quartz tube reactor
that was mounted into a two-zone tube furnace. The furnace was 32
inches in overall length with each zone 16 inches in length. The
spool of yarn was placed into a container fitted with a nitrogen
gas inlet that was sealed to the quartz reaction tube at the
"upstream" end of the tube such that the yarn was fed down the tube
reactor through the furnaces and used to pull the continuous
treated AU4-12K yarn through. In this way the inert nitrogen gas
purges the yarn chamber and then flows down the reaction tube
through the furnace and out the downstream end open to the
atmosphere. The first zone of the furnace was set to a temperature
of 800.degree. C. and the second set to a temperature of
820.degree. C. Nitrogen flow was set to 500 sccm in the reactor
system and maintained throughout the entire process. After a period
of 15 minutes to allow the reactor system to completely purge the
yarn was pulled through the entire length of the furnaces at a
speed of 1.27 cm/minute. When the free end of the AU4-12K yarn had
just entered the tube furnace high purity acetylene was introduced
upstream of the tube furnace at a flow rate of 5.0 sccm (1%
concentration of acetylene in nitrogen). This process created a
gradient of heat treatment and growth exposure times along the 60
cm length of the catalyst treated AU4-12K yarn in the tube furnace.
When the AU4-12K yarn exited the tube furnace the acetylene flow
was stopped and the reactor cooled under nitrogen purge.
[0182] Carbon nanotubes were found to grow from the surfaces of the
individual filaments in the AU4-12K yarn along the 60 cm length.
The surface density and the characteristics of the carbon nanotubes
varied depending on the length of time each segment spent
respectively in the zones of the furnaces and the length of growth
time with acetylene present. In FIGS. 11A and 11B the scanning
electron microscope images show the nanotube growth for the segment
of AU4-12K 46 cm from the leading edge (FIG. 11A) and 5 cm from the
leading edge (FIG. 11B).
Example 10
Continuous Growth of Carbon Nanotubes at Atmospheric Pressure on a
30 Meter Length of Silicon Carbide Fiber (Hi-Nicalon.TM.) with an
Additive to Catalyst Solution to Improve Wetting
[0183] A 5 wt % solution of Benzophenone-3,3',4,4'-tetracarboxylic
dianhydride (BTDA) in anhydrous ethanol was refluxed for 2 hours to
convert the BTDA to a soluble diethyl ester-acid derivative (BTDE).
This solution was subsequently diluted with acetone to 0.5 wt %
solution. A 100 mM Ferric Nitrate Nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) catalyst solution was prepared using
this BTDE-ethanol-acetone solution and allow to stand for 24 hours.
A 30 m length of as-received silicon carbide ceramic fiber yarn was
used (Hi-Nicalon.TM.). The yarn length was level wound onto a glass
spool and then continuously run through a sonicated bath of the
catalyst solution with a residence time of approximately 10 seconds
then immediately through a second bath of hexanes and dried in air
at room temperature, then level wound onto a glass spool. The
carbon nanotube growth on the continuous yarn was conducted in a 25
mm diameter quartz tube reactor 250 cm in length that was mounted
into two separate tube furnaces with a 50 cm gap between the
furnaces. Both tube furnaces were 40 cm in overall length. The
spool of yarn was placed into a container fitted with a nitrogen
gas inlet that was sealed to the quartz reaction tube at the
"upstream" end of the tube such that the yarn was fed down the tube
reactor through the furnaces and used to pull the continuous
treated silicon carbide (Hi-Nicalon.TM.) yarn through. In this way
the inert nitrogen gas purges the yarn chamber and then flows down
the reaction tube through the furnaces and out the downstream end
open to the atmosphere. The first tube furnace was set to a
temperature of 800.degree. C. and the second set to a temperature
of 820.degree. C. Nitrogen flow was set to 500 sccm in the reactor
system and maintained throughout the entire process. Acetylene was
introduced through a gas fitting in the quartz tube in the gap
between the tube furnaces at a flow rate of 5.0 sccm (1%
concentration of acetylene in nitrogen). After a period of 15
minutes to allow the reactor system to completely purge, the yarn
was pulled through the entire length of quartz tube through the
furnaces at a speed of 1.27 cm/minute and level wound onto a glass
spool. After a period of approximately 8 hours; 600 cm of the
silicon carbide yarn (Hi-Nicalon.TM.) had been pulled through the
reactor. The acetylene was shut off and the furnaces allowed to
cool under nitrogen flow with the silicon carbide yarn
(Hi-Nicalon.TM.) still in the reactor. Approximately 16 hours
later, the furnaces were again heated to 800.degree. C. and
820.degree. C. respectively, 5.0 sccm acetylene flow started again
and the preform yarn (Hi-Nicalon.TM.) pulled through at a rate of
1.27 cm/min for approximately 8 hours. This cycle was repeated each
day for a total of 5 days until the entire 30 m length of preform
yarn (Hi-Nicalon.TM.) was continuously run through the carbon
nanotube reactor system
[0184] Carbon nanotubes were found to grown from the surfaces of
the individual filaments in the preform yarn along its 30 m length.
In FIG. 12 the scanning electron microscope images show the carbon
nanotube growth on a segment of yarn from the middle of the 30 m
length of preform yarn.
Example 11
Growth of Carbon Nanotubes at Atmospheric Pressure on a 105 cm
Length of Ceramic Grade Silicon Carbide Fiber Yarn (CG-Nicalon.TM.)
Resulting in Higher Electrical Conductivity.
[0185] A 100 mM Ferric Nitrate Nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) solution in an acetone and ethanol
mixture (50/50) was prepared and allowed to stand for 24 hours. A
105 cm length of as-received ceramic grade silicon carbide fiber
yarn (CG-Nicalon.TM.) was used for the continuous CNT growth
process. The ceramic grade silicon carbide fiber yarn was soaked in
the catalyst solution for 5 minutes and hung vertically to dry at
room temperature. The treated ceramic grade silicon carbide fiber
yarn preform was then placed into 50 mm quartz tube in a tube
furnace at 500.degree. C. under a nitrogen flow of 500 sccm for
pyrolysis of the catalyst for a residence time of 15 minutes and
the "downstream" end of the quartz tube was open to the atmosphere.
The carbon nanotube growth on the continuous yarn was conducted in
a 25 mm quartz tube reactor that was mounted into a two-zone tube
furnace. The furnace was 80 cm in overall length with each zone 40
cm in length. The catalyst treated end of the yarn was placed into
a container fitted with a nitrogen gas inlet that was sealed to the
quartz reaction tube at the upstream end of the tube. In this way,
the inert nitrogen gas purges the yarn chamber and then flows down
the reaction tube through the furnace and out the downstream end
open to the atmosphere. The first zone of the furnace was set to a
temperature of 800.degree. C. and the second set to a temperature
of 820.degree. C. Nitrogen flow was set to 500 sccm in the reactor
system and maintained throughout the entire process. After a period
of 15 minutes, to allow the reactor system to completely purge, the
yarn was pulled into the first tube furnace section and held there
for 15 minutes. The treated yarn length was then pulled into the
second tube furnace and high purity acetylene was introduced
upstream of the tube furnace at a flow rate of 5.0 sccm (1%
concentration of acetylene in nitrogen) and kept flowing for 15
minutes. After 15 minutes the acetylene flow was stopped, the
treated yarn was pulled out of the second tube furnace and allowed
to cool under nitrogen purge for 5 minutes before being pulled out
of the quartz reactor tube into the atmosphere
[0186] Carbon nanotubes were found to grow from the surfaces of the
individual filaments in the ceramic grade silicon carbide fiber
yarn preform along its entire 15 cm length. In FIGS. 13A and 13B
SEM images show the nanotube growth for the ceramic grade silicon
carbide fiber yarn. The electrical resistance of the yarn was found
to decrease from 2.88.times.10.sup.8 .OMEGA./cm to
5.00.times.10.sup.3 .OMEGA./cm after carbon nanotube growth
demonstrating the dramatic enhancement in conductivity from the in
situ formation of carbon nanotubes. FIGS. 13A and B are the same
preparation, at 350.times., and 1300.times. magnification
respectively.
Example 12
Continuous Growth of Carbon Nanotubes at Atmospheric Pressure on a
5 Meter Length of Ceramic Grade Silicon Carbide Fiber Yarn and
Fabrication of a Polyimide Matrix Composite with Improved Fracture
Toughness
[0187] A 100 mM Ferric Nitrate Nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) solution in isopropanol was prepared
and allowed to stand for 24 hours. A 5 m length of as-received
ceramic grade silicon carbide fiber yarn. The 5 m length of yarn
was level wound onto a glass spool and dipped into a bath of the
catalyst solution with a residence time of approximately 1 minute
then immediately dipped into a second bath of hexanes and dried in
air at room temperature. The carbon nanotube growth on the
continuous yarn was conducted in 25 mm diameter quartz tube reactor
250 cm in length that was mounted into two separate tube furnaces
with a 50 cm gap between the furnaces. Both tube furnaces were 40
cm in overall length. The spool of yarn was placed into a container
fitted with a nitrogen gas inlet that was sealed to the quartz
reaction tube at the "upstream" end of the tube such that the yarn
was fed down the tube reactor through the furnaces and used to pull
the continuous treated ceramic grade silicon carbide fiber yarn
through. In this way the inert nitrogen gas purges the yarn chamber
and then flows down the reaction tube through the furnaces and out
the downstream end open to the atmosphere. The first tube furnace
was set to a temperature of 800.degree. C. and the second set to a
temperature of 820.degree. C. Nitrogen flow was set to 500 sccm in
the reactor system and maintained throughout the entire process.
Acetylene was introduced through a gas fitting in the quartz tube
in the gap between the tube furnaces at a flowrate of 5.0 sccm (1%
concentration of acetylene in nitrogen). After a period of 15
minutes, to allow the reactor system to completely purge, the yarn
was pulled through the entire length of quartz tube through the
furnaces at a speed of 1.27 cm/minute and level wound onto a glass
spool. After a period of approximately 7 hours the entire 500 cm
length of ceramic grade silicon carbide fiber yarn had been pulled
through the reactor.
[0188] Carbon nanotubes were found to grown from the surfaces of
the individual filaments in the ceramic grade silicon carbide fiber
yarn along its 5 m length. The 5 m length of yarn was used to
fabricate a polyimide matrix composite material using P.sup.2SI.TM.
635LM (Performance Plolymer Solutions, Inc.) commercial resin
solution loaded with carbon particles by compression molding. The
unidirectional pre-ceramic composite had a fiber volume fraction of
approximately 20%. Interlaminar shear strength was measured
according to ASTM D2344. Carbon nanotube induced ply interlocking
was observed in the load-displacement curve, as shown in FIG. 14,
compared to an equivalent composite fabricated with as-received
ceramic grade silicon carbide fiber yarn. As shown in FIG. 14, the
nanotube treated yarn composite exhibited a higher stiffness,
higher elongation to failure (over 200%), and 55% higher fracture
toughness (area under the load-displacement curve of 399 N-mm for
the untreated ceramic grade silicon carbide fiber yarn control
compared to 617 N-mm using nanotube treated yarn, an increase of
approximately 55%).
Example 13
Continuous Growth of Carbon Nanotubes at Atmospheric Pressure on a
5 Meter Length of Ceramic Grade Silicon Carbide Fiber Yarn and
Fabrication of a Ceramic Matrix Composite
[0189] A 100 mM Ferric Nitrate Nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) solution in isopropanol was prepared
and allowed to stand for 24 hours. A 5 m length of as-received
ceramic grade silicon carbide fiber yarn was used. The 5 m length
of yarn was level wound onto a glass spool and dipped into a bath
of the catalyst solution with a residence time of approximately 1
minute then immediately dipped into a second bath of hexanes and
dried in air at room temperature. The carbon nanotube growth on the
continuous yarn was conducted in a 25 mm diameter quartz tube
reactor 250 cm in length that was mounted into two separate tube
furnaces with a 50 cm gap between the furnaces. Both tube furnaces
were 40 cm in overall length. The spool of yarn was placed into a
container fitted with a nitrogen gas inlet that was sealed to the
quartz reaction tube at the "upstream" end of the tube such that
the yarn was fed down the tube reactor through the furnaces and
used to pull the continuous treated ceramic grade silicon carbide
fiber yarn through. In this way the inert nitrogen gas purges the
yarn chamber and then flows down the reaction tube through the
furnaces and out the downstream end open to the atmosphere. The
first tube furnace was set to a temperature of 800.degree. C. and
the second set to a temperature of 820.degree. C. Nitrogen flow was
set to 500 sccm in the reactor system and maintained throughout the
entire process. Acetylene was introduced through a gas fitting in
the quartz tube in the gap between the tube furnaces at a flowrate
of 5.0 sccm (1% concentration of acetylene in nitrogen). After a
period of 15 minutes, to allow the reactor system to completely
purge, the yarn was pulled through the entire length of quartz tube
through the furnaces at a speed of 1.27 cm/minute and level wound
onto a glass spool. After a period of approximately 7 hours the 50
m length of ceramic grade silicon carbide fiber yarn had been
pulled through the reactor.
[0190] Carbon nanotubes were found to grown from the surfaces of
the individual filaments within the ceramic grade silicon carbide
fiber yarn and along its 5 m length. In FIGS. 15A and 15B below the
SEM images show the nanotube growth on the 5 m length of ceramic
grade silicon carbide fiber yarn. The 5 m length of yarn was used
to fabricate a pre-ceramic polyimide matrix composite
(unidirectional, fiber volume fraction of approximately 20%). The
pre-ceramic composite was heated under flowing nitrogen to
800.degree. C. to pyrolyze the matrix to carbon. Afterwards, the
green body was melt infiltrated with pure silicon at 1500.degree.
C. under vacuum to form the ceramic matrix composite article. FIGS.
15A and B displays the ceramic matrix composite article fabricated
by reactive melt infiltration. FIG. 15B is the same preparation as
FIG. 15A but at a higher magnification.
Example 14
Continuous Growth of Carbon Nanotubes at Atmospheric Pressure on a
5 Meter Length of Ceramic Grade Silicon Carbide Fiber Yarn and
Subsequent Purification
[0191] A 100 mM Ferric Nitrate Nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) solution in isopropanol was prepared
and allowed to stand for 24 hours. A 5 m length of as-received
ceramic grade silicon carbide fiber yarn (Nicalon.TM. CG) was used.
The 5 m length of yarn was level wound onto a glass spool and
dipped into a bath of the catalyst solution with a residence time
of approximately 1 minute then immediately dipped into a second
bath of hexanes and dried in air at room temperature. The carbon
nanotube growth on the continuous yarn was conducted in a 25 mm
diameter quartz tube reactor 250 cm in length that was mounted into
two separate tube furnaces with a 50 cm gap between the furnaces.
Both tube furnaces were 40 cm in overall length. The spool of yarn
was placed into a container fitted with a nitrogen gas inlet that
was sealed to the quartz reaction tube at the "upstream" end of the
tube such that the yarn was fed down the tube reactor through the
furnaces and used to pull the continuous treated ceramic grade
silicon carbide fiber yarn (Nicalon.TM. CG) through. In this way
the inert nitrogen gas purges the yarn chamber and then flows down
the reaction tube through the furnaces and out the downstream end
open to the atmosphere. The first tube furnace was set to a
temperature of 800.degree. C. and the second set to a temperature
of 820.degree. C. Nitrogen flow was set to 500 sccm in the reactor
system and maintained throughout the entire process. Acetylene was
introduced through a gas fitting in the quartz tube in the gap
between the tube furnaces at a flowrate of 5.0 sccm (1%
concentration of acetylene in nitrogen). After a period of 15
minutes, to allow the reactor system to completely purge, the yarn
was pulled through the entire length of quartz tube through the
furnaces at a speed of 1.27 cm/minute and level wound onto a glass
spool. After a period of approximately 7 hours the 5 m length of
ceramic grade silicon carbide fiber yarn (Nicalon.TM. CG) had been
pulled through the reactor.
[0192] Carbon nanotubes were found to grown from the surfaces of
the individual filaments in the ceramic grade silicon carbide fiber
yarn along its 5 m length. The 5 m length of yarn wound on a glass
spool was immersed into a flask of concentrated nitric acid and
heated to reflux for 1 hour to clean the carbon nanotube treated
yarn of residual metal catalyst and amorphous carbon deposits. The
yarn was subsequently examined and the carbon nanotube growth
covering the individual filaments of the ceramic grade silicon
carbide fiber yarn was found to be substantially free of amorphous
carbon and the carbon nanotubes were intact. FIG. 16A shows a
segment of the yarn at 950.times. magnification. FIG. 16B shows the
same preparation at 2,400.times. magnification.
Example 15
Continuous Growth of Carbon Nanotubes at Atmospheric Pressure on a
5 Meter Length of Ceramic Grade Silicon Carbide Fiber Yarn Using
Cobalt Acetate Catalyst Precursor Solution
[0193] A 100 mM Cobalt nitrate, Co(NO.sub.3).sub.2.6H.sub.2O,
solution in water was prepared and allowed to stand for 24 hours. A
5 m length of as-received ceramic grade silicon carbide fiber yarn
(Nicalon.TM. CG). The 5 m length of yarn was level wound onto a
glass spool and dipped into a bath of the catalyst solution with a
residence time of approximately 1 minute and allowed to dry at room
temperature. The carbon nanotube growth on the continuous yarn was
conducted in a 25 mm diameter quartz tube reactor 250 cm in length
that was mounted into two separate tube furnaces with a 50 cm gap
between the furnaces. Both tube furnaces were 40 cm in overall
length. The spool of yarn was placed into a container fitted with a
nitrogen gas inlet that was sealed to the quartz reaction tube at
the "upstream" end of the tube such that the yarn was fed down the
tube reactor through the furnaces and used to pull the continuous
treated ceramic grade silicon carbide fiber yarn through. In this
way the inert nitrogen gas purges the yarn chamber and then flows
down the reaction tube through the furnaces and out the downstream
end open to the atmosphere. The first tube furnace was set to a
temperature of 800.degree. C. and the second set to a temperature
of 820.degree. C. Nitrogen flow was set to 500 sccm in the reactor
system and maintained throughout the entire process. Acetylene was
introduced through a gas fitting in the quartz tube in the gap
between the tube furnaces at a flowrate of 5.0 sccm (1%
concentration of acetylene in nitrogen). After a period of 15
minutes, to allow the reactor system to completely purge, the yarn
was pulled through the entire length of quartz tube through the
furnaces at a speed of 1.27 cm/minute and level wound onto a glass
spool. After a period of approximately 7 hours the 5 m length of
Nicalon.TM. CG had been pulled through the reactor.
[0194] Carbon nanotubes were found to grown from the surfaces of
the individual filaments in the ceramic grade silicon carbide fiber
yarn along its 5 m length as shown in FIGS. 17A and 17B. The carbon
nanotube filaments were uniformly dispersed along the ceramic
fibers in the yarn.
Example 16
Growth of Carbon Nanotubes on Continuous Preform Materials Using
Continuous Growth Process at a Variety of Variables
[0195] Tables 2 and 3 illustrate the use of the invention using a
variety variables. Table 2 shows the use of the invention with a
variety of catalysts, solvents, concentrations and drying
temperatures. Table 3 shows the use of the invention PVAC and BTDE
additives to enhance the CNT growth.
TABLE-US-00003 TABLE 2 Catalyst Trials on Continuous Ceramic Yarn
CNT Processes Concentration Drying Duration of Metal Salt Solvent
(mM) temperature aging Ferric nitrate Water 50 and 100 Room <1
hour, 24 Fe(NO.sub.3).sub.3.cndot.9H.sub.2O temperature and hours
70.degree. C. Ferric nitrate Ethyl Alcohol 50 and 100 Room <1
hour, 24 Fe(NO.sub.3).sub.3.cndot.9H.sub.2O temperature and hours
70.degree. C. Ferric nitrate Isopropyl 50 and 100 Room <1 hour,
24 Fe(NO.sub.3).sub.3.cndot.9H.sub.2O Alcohol and temperature and
hours, 5 hours, post-dip in 70.degree. C. >48 hours hexanes
Ferric nitrate Isopropyl 100 Room <1 hour, 24
Fe(NO.sub.3).sub.3.cndot.9H.sub.2O alcohol and temperature and
hours without post- 70.degree. C. dip in hexanes Cobalt nitrate
Water 50 and 100 Room <1 hour, 24
Co(NO.sub.3).sub.2.cndot.6H.sub.2O temperature and hours 70.degree.
C. Cobalt nitrate Ethyl alcohol 50 and 100 Room <1 hour, 24
Co(NO.sub.3).sub.2.cndot.6H.sub.2O temperature and hours 70.degree.
C. Cobalt nitrate Isopropyl 50 and 100 Room <1 hour, 24
Co(NO.sub.3).sub.2.cndot.6H.sub.2O alcohol and temperature and
hours post-dip in 70.degree. C. hexanes Cobalt nitrate Isopropyl 50
and 100 Room <1 hour, 24 Co(NO.sub.3).sub.2.cndot.6H.sub.2O
alcohol and temperature and hours without post- 70.degree. C. dip
in hexanes Nickel nitrate Water 100 Room <1 hour, 24
Ni(NO.sub.3).sub.2.cndot.6H.sub.2O temperature and hours 70.degree.
C. Nickel nitrate Ethyl alcohol 100 Room <1 hour, 24
Ni(NO.sub.3).sub.2.cndot.6H.sub.2O temperature and hours 70.degree.
C. Nickel nitrate Isopropyl 100 Room <1 hour, 24
Ni(NO.sub.3).sub.2.cndot.6H.sub.2O alcohol and temperature and
hours post-dip in 70.degree. C. hexanes Nickel nitrate Isopropyl
100 Room <1 hour, 24 Ni(NO.sub.3).sub.2.cndot.6H.sub.2O alcohol
and temperature and hours without post- 70.degree. C. dip in
hexanes
TABLE-US-00004 TABLE 3 Test matrix of experiments conducted with
PVAC and BTDE additives to catalyst solution to improve wetting of
Nicalon .TM. and carbon fiber yarns. Spec- imen Wt % ID Additive in
No. Additive Solution Catalyst solution 1 BTDE.sup.1 5.0/IPA 100 mM
Fe(NO.sub.3).sub.3/IPA, no hexanes 2 BTDE 5.0/IPA 100 mM
Fe(NO.sub.3).sub.3/IPA, with hexanes 3 BTDE 5.0/IPA 100 mM
Fe(NO.sub.3).sub.3/acetone 4 BTDE 0.5/IPA 100 mM
Fe(NO.sub.3).sub.3/IPA, no hexanes 5 BTDE 0.5/IPA 100 mM
Fe(NO.sub.3).sub.3/IPA, with hexanes 6 BTDE 0.5/IPA 100 mM
Fe(NO.sub.3).sub.3/acetone 7 BTDE 0.5/Acetone 100 mM
Fe(NO.sub.3).sub.3/IPA, no hexanes 8 BTDE 0.5/Acetone 100 mM
Fe(NO.sub.3).sub.3/IPA, with hexanes 9 BTDE 0.5/Acetone 100 mM
Fe(NO.sub.3).sub.3/acetone 10 PVAC.sup.2 1.0/Acetone 100 mM
Fe(NO.sub.3).sub.3/acetone, no hexanes 11 PVAC 0.5/Acetone 100 mM
Fe(NO.sub.3).sub.3/acetone, no hexanes 12 PVAC 5.0/IPA 100 mM
Fe(NO.sub.3).sub.3/acetone 13 PVAC 1.0/Acetone 100 mM
Fe(NO.sub.3).sub.3/IPA, no hexanes 14 PVAC 1.0/Acetone 100 mM
Fe(NO.sub.3).sub.3/IPA, with hexanes .sup.1dimethylester of
3,3',4,4'-benzo-phenonetetracarboxylic acid .sup.2PolyVinyl
Acetate
Example 17
Continuous Growth of Carbon Nanotubes on a 60 cm Length of Silicon
Carbide Fiber Yarn (Hi-Nicalon.TM.) at Atmospheric Pressure Without
Heat Treatment
[0196] A 100 mM Ferric Nitrate Nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) solution in an acetone and ethanol
mixture (50/50) was prepared and allowed to stand for 24 hours. A
60 cm length of as-received silicon carbide fiber yarn was used for
the continuous CNF growth process. The silicon carbide fiber yarn
was soaked in the catalyst solution for 5 minutes and hung
vertically to dry at room temperature. The dry catalyst treated
yarn was then bonded to a 305 cm length of untreated ceramic grade
silicon carbide fiber yarn (Nicalon.TM. CG) that served as a leader
to continuously pull yarn through the reactor at a controlled
speed. The treated silicon carbide fiber yarn was then placed into
a 25 mm diameter quartz tube in a tube furnace at 500.degree. C.
under a nitrogen flow of 500 sccm for pyrolysis of the catalyst at
a residence time of 15 minutes and the "downstream" end of the
quartz tube was open to the atmosphere. The carbon nanotube growth
on the continuous yarn was conducted in a 25 mm diameter quartz
tube reactor that was mounted in a tube furnace. The furnace was 80
cm in overall length. The spool of yarn was placed into a container
fitted with a nitrogen gas inlet that was sealed to the quartz
reaction tube at the upstream end of the tube such that the
untreated ceramic grade silicon carbide fiber leader was fed down
the tube reactor through the furnaces to be used to pull the
continuous treated silicon carbide fiber yarn through. In this way
the inert nitrogen gas purges the yarn chamber and then flows down
the reaction tube through the furnace and out the downstream end
open to the atmosphere. The furnace was set to a temperature of
820.degree. C. Nitrogen flow was set to 500 sccm in the reactor
system and acetylene flow was set to 5 sccm and maintained
throughout the entire process. After a period of 15 minutes to
allow the reactor system to completely purge, the pyrolyzed
catalyst treated yarn was pulled through the entire length of the
furnaces at a speed of 0.635 cm/minute corresponding to a residence
time of 92 minutes in the tube furnace reactor. When the
Hi-Nicalon.RTM. yarn exited the tube furnace, the acetylene flow
was stopped and the reactor cooled under nitrogen purge.
[0197] Carbon nanotubes were found uniformly grown from the
surfaces of the individual filaments in the silicon carbide fiber
yarn along the entire length of the yarn. In FIGS. 18A and 18B, the
scanning electron microscope images show the nanotube growth for
the segment of silicon carbide fiber 58.4 cm from the leading edge
at 540.times. magnification (FIG. 18A) and 58.4 cm from the leading
edge at 750.times. magnification (FIG. 18B).
Example 18
Continuous Growth of Carbon Nanotubes on Multiple Silicon Carbide
Fiber Continuous Yarns Simultaneously at Atmospheric Pressure
Without a Separate Pyrolysis Process Step to Demonstrate Scale-Up
of the Manufacturing Process
[0198] A 100 mM Ferric Nitrate Nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) solution in ethanol mixture was
prepared and allowed to stand for 24 hours. Three 5 m lengths of
as-received silicon carbide (SiC) fiber (Nicalon.TM. CG) yarn were
used simultaneously for the continuous CNT growth process. The
individual 5 m SiC yarns were level wound onto 25 mm diameter glass
spools and soaked in the catalyst solution for 5 minutes then
placed in a convection oven at 50.degree. C. until dry. The carbon
nanotube growth on the multiple continuous yarns was conducted in a
25 mm diameter quartz tube reactor, 1830 cm in overall length, that
was mounted into two separate tube furnaces (40 cm in overall
length each) with a 50 cm space between the furnaces. The entrance
end of the quartz tube was fitted with an adapter with three 5 mm
diameter holes equally spaced in across the diameter at the
midpoint of the tube. Each hole was covered with a silicone rubber
septum that was slit to allow the yarn to enter but keep air out of
the tube furnace. The three spools of catalyst precursor treated
SiC yarn were mounted on a spindle before the entrance to the
quartz tube to allow rotation and to allow the yarn to be unwound
as it was pulled continuously through the tube furnace reactor
set-up continuously at a controlled speed. The SiC yarns were
threaded through the septa, through the first tube furnace, through
an aluminum bushing located in the quartz tube between the two
furnaces with three equally spaced holes across the diameter at the
midpoint of the tube ID, through the second tube furnace and
attached to a level winding mechanism that pulled the three yarns
through the quartz tube a controlled rate. The first tube furnace
was set to a temperature of 800.degree. C. and the second tube
furnace was set to a temperature of 820.degree. C. Nitrogen was
introduced into a fitting at the yarn entrance to the quartz tube
and flow was set to 500 sccm and maintained throughout the entire
process. Acetylene was introduced into a fitting in the quartz tube
between the tube furnaces and the flow was set to 2 sccm and
maintained throughout the entire process. After a period of 15
minutes to allow the reactor system to completely purge, the
catalyst treated yarns were pulled through the entire length of the
furnaces at a speed of 1.27 cm/minute corresponding to a residence
time of 31.5 minutes in the tube furnace reactor. After a period of
operation for approximately 6.5 hours, when the SiC yarns exited
the tube furnace, the acetylene flow was stopped and the reactor
cooled under nitrogen purge.
[0199] Carbon nanofibers were found uniformly grown from the
surfaces of the individual filaments in the yarns along the entire
length of the yarns that were pulled through the entire process
set-up (approximately the first 1 m of SiC yarn did not have VGCNT
since it was used as the leader to pull the yarns through).
[0200] Those of skill in the art will appreciate that, while the
above example discloses the use of multiple continuous fiber yarns,
of the same type, processed concurrently, it is within the scope of
the invention to process different types of continous fiber
preforms concurrently. Thus, for example, the yarns may include
silicon carbide, quartz, metal, glass or ceramic, without
limitation. Further, it should be appreciated that by "processed
together" the multiple yarns may undergo the entire process of
nanotube growth, e.g., dispersal of catalyst precursor; conversion
of the catalyst precursor
[0201] While this invention has been described in conjunction with
the various exemplary embodiments outlined above, various
alternatives, modifications, variations, improvements, and/or
substantial equivalents, whether known or that are or may be
presently unforeseen, may become apparent to those having at least
ordinary skill in the art. Accordingly, the exemplary embodiments
according to this invention, as set forth above, are intended to be
illustrative, not limiting. Various changes may be made without
departing from the spirit and scope of the invention. Therefore,
the invention is intended to embrace all known or later-developed
alternatives, modifications, variations, improvements, and/or
substantial equivalents of these exemplary embodiments
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